zlib: move constants into zlib.constants

zlib constants were previously being added to binding in node_zlib.cc.
This moves the zlib constants to node_constants.cc for consistency with
the recent constants refactoring:
  #6534

Adds require('zlib').constants to expose the constants
Docs-only deprecates the constants hung directly off require('zlib')
Removes a couple constants from the docs that apparently no longer
exist in the code

PR-URL: #7203
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/zlib.md

@@ -123,7 +123,9 @@ method that is used to compressed the last chunk of input data:
 // This is a truncated version of the buffer from the above examples
 const buffer = Buffer.from('eJzT0yMA', 'base64');
 
-zlib.unzip(buffer, { finishFlush: zlib.Z_SYNC_FLUSH }, (err, buffer) => {
+zlib.unzip(buffer,
+           {finishFlush: zlib.constants.Z_SYNC_FLUSH},
+           (err, buffer) => {
   if (!err) {
     console.log(buffer.toString());
   } else {
@@ -221,65 +223,56 @@ added: v0.5.8
 
 <!--type=misc-->
 
-All of the constants defined in `zlib.h` are also defined on `require('zlib')`.
-In the normal course of operations, it will not be necessary to use these 
-constants. They are documented so that their presence is not surprising. This 
-section is taken almost directly from the [zlib documentation][].  See
-<http://zlib.net/manual.html#Constants> for more details.
+All of the constants defined in `zlib.h` are also defined on
+`require('zlib').constants`. In the normal course of operations, it will not be
+necessary to use these  constants. They are documented so that their presence is
+not surprising. This section is taken almost directly from the
+[zlib documentation][].  See <http://zlib.net/manual.html#Constants> for more
+details.
+
+*Note*: Previously, the constants were available directly from 
+`require('zlib')`, for instance `zlib.Z_NO_FLUSH`. Accessing the constants
+directly from the module is currently still possible but should be considered
+deprecated.
 
 Allowed flush values.
 
-* `zlib.Z_NO_FLUSH`
-* `zlib.Z_PARTIAL_FLUSH`
-* `zlib.Z_SYNC_FLUSH`
-* `zlib.Z_FULL_FLUSH`
-* `zlib.Z_FINISH`
-* `zlib.Z_BLOCK`
-* `zlib.Z_TREES`
+* `zlib.constants.Z_NO_FLUSH`
+* `zlib.constants.Z_PARTIAL_FLUSH`
+* `zlib.constants.Z_SYNC_FLUSH`
+* `zlib.constants.Z_FULL_FLUSH`
+* `zlib.constants.Z_FINISH`
+* `zlib.constants.Z_BLOCK`
+* `zlib.constants.Z_TREES`
 
 Return codes for the compression/decompression functions. Negative
 values are errors, positive values are used for special but normal
 events.
 
-* `zlib.Z_OK`
-* `zlib.Z_STREAM_END`
-* `zlib.Z_NEED_DICT`
-* `zlib.Z_ERRNO`
-* `zlib.Z_STREAM_ERROR`
-* `zlib.Z_DATA_ERROR`
-* `zlib.Z_MEM_ERROR`
-* `zlib.Z_BUF_ERROR`
-* `zlib.Z_VERSION_ERROR`
+* `zlib.constants.Z_OK`
+* `zlib.constants.Z_STREAM_END`
+* `zlib.constants.Z_NEED_DICT`
+* `zlib.constants.Z_ERRNO`
+* `zlib.constants.Z_STREAM_ERROR`
+* `zlib.constants.Z_DATA_ERROR`
+* `zlib.constants.Z_MEM_ERROR`
+* `zlib.constants.Z_BUF_ERROR`
+* `zlib.constants.Z_VERSION_ERROR`
 
 Compression levels.
 
-* `zlib.Z_NO_COMPRESSION`
-* `zlib.Z_BEST_SPEED`
-* `zlib.Z_BEST_COMPRESSION`
-* `zlib.Z_DEFAULT_COMPRESSION`
+* `zlib.constants.Z_NO_COMPRESSION`
+* `zlib.constants.Z_BEST_SPEED`
+* `zlib.constants.Z_BEST_COMPRESSION`
+* `zlib.constants.Z_DEFAULT_COMPRESSION`
 
 Compression strategy.
 
-* `zlib.Z_FILTERED`
-* `zlib.Z_HUFFMAN_ONLY`
-* `zlib.Z_RLE`
-* `zlib.Z_FIXED`
-* `zlib.Z_DEFAULT_STRATEGY`
-
-Possible values of the data_type field.
-
-* `zlib.Z_BINARY`
-* `zlib.Z_TEXT`
-* `zlib.Z_ASCII`
-* `zlib.Z_UNKNOWN`
-
-The deflate compression method (the only one supported in this version).
-
-* `zlib.Z_DEFLATED`
-
-For initializing zalloc, zfree, opaque.
-
-* `zlib.Z_NULL`
+* `zlib.constants.Z_FILTERED`
+* `zlib.constants.Z_HUFFMAN_ONLY`
+* `zlib.constants.Z_RLE`
+* `zlib.constants.Z_FIXED`
+* `zlib.constants.Z_DEFAULT_STRATEGY`
 
 ## Class Options
 <!-- YAML
@@ -293,8 +286,8 @@ Each class takes an `options` object.  All options are optional.
 Note that some options are only relevant when compressing, and are
 ignored by the decompression classes.
 
-* `flush` (default: `zlib.Z_NO_FLUSH`)
-* `finishFlush` (default: `zlib.Z_FINISH`)
+* `flush` (default: `zlib.constants.Z_NO_FLUSH`)
+* `finishFlush` (default: `zlib.constants.Z_FINISH`)
 * `chunkSize` (default: 16*1024)
 * `windowBits`
 * `level` (compression only)
@@ -368,7 +361,7 @@ class of the compressor/decompressor classes.
 added: v0.5.8
 -->
 
-`kind` defaults to `zlib.Z_FULL_FLUSH`.
+`kind` defaults to `zlib.constants.Z_FULL_FLUSH`.
 
 Flush pending data. Don't call this frivolously, premature flushes negatively
 impact the effectiveness of the compression algorithm.
@@ -391,6 +384,10 @@ Only applicable to deflate algorithm.
 added: v0.7.0
 -->
 
+## zlib.constants
+
+Provides an object enumerating Zlib-related [constants][].
+
 Reset the compressor/decompressor to factory defaults. Only applicable to
 the inflate and deflate algorithms.
 
@@ -545,3 +542,4 @@ Decompress a Buffer or string with Unzip.
 [Unzip]: #zlib_class_zlib_unzip
 [`.flush()`]: #zlib_zlib_flush_kind_callback
 [Buffer]: buffer.html
+[constants]: #constants_constants