Use RustLSP library.

A more extensive and principled way to handle the LSP protocol. Adds
*interface* support for all LSP requests.

Fixes:
* 'completionItem/resolve' should return CompletionItem not a vector
* Concurrent handling of requests. Requests must be handled in order,
otherwise, for example, text document changes could be applied out of
order. Only read-only requests can conceptually be executed in parallel
(since that would have the same result as if they were processed in
order). See here for more info:
microsoft/language-server-protocol#12

TODO for RustLSP:
 * Cancel support

Cargo.toml

@@ -7,7 +7,7 @@ authors = ["Jonathan Turner <jturner@mozilla.com>"]
 cargo = { path = "../cargo" }
 derive-new = "0.3"
 env_logger = "0.3"
-languageserver-types = { version = "0.5.0", git = "https://github.com/gluon-lang/languageserver-types" }
+rust_lsp = { version = "0.6.0", git = "https://github.com/RustDT/RustLSP" }
 log = "0.3"
 racer = { git = "https://github.com/phildawes/racer" }
 rls-span = { git = "https://github.com/nrc/rls-span" }