split low-level API into separate module (#68)

The low-level API wrappers now sit in a submodule LibSerialPort.wrap.

The high-level API currently still re-exports a few symbols from that
low-level API module, to preserve backwards compatibility.

This is due to

* my previously started attempt to merge the low and high-level
  APIs for the three functions sp_flush, sp_drain, and sp_output_waiting

* the fact that several high-level APIs currently still refer to
  enum types and constants defined by the low-level API

We may want to phase out either or both in future, for a cleaner
separation, and a more Julian high-level API.

docs/make.jl

@@ -1,3 +1,8 @@
-using Documenter, LibSerialPort
+using Documenter, LibSerialPort, LibSerialPort.wrap
 
-makedocs(sitename="LibSerialPort.jl")
+makedocs(
+    sitename="LibSerialPort.jl",
+    format = Documenter.HTML(
+        prettyurls = get(ENV, "CI", nothing) == "true"
+    )
+)

docs/src/wrap.md

@@ -0,0 +1,5 @@
+# Low-level C API wrappers
+
+```@autodocs
+Modules = [LibSerialPort.wrap]
+```