add --wait stdio

[nadako]

This PR adds support for --wait stdio argument which makes haxe completion server use stdin and stderr instead of sockets for communication.

This solves a number of annoying problems for IDE, for example:

• multiple instances of an IDE don't need to somehow determine a free port to use for a new haxe server
• we don't need to care about other programs already listening to the port we'd like to use for haxe
• when starting haxe server as a subprocess, we don't need to wait/retry connecting to the server and can invoke queries right away without any race conditions (e.g. https://github.com/nadako/haxe-languageserver/issues/7)

The protocol is the following:

• everything is binary
• ide sends queries to stdin, haxe replies to stderr
• ide and haxe exchange messages consiting of two parts: data length (4 byte little-endian int) and data
• data is the same as with socket - newline separated hxml argumens and optional stdin display file content, (separated by \x01).

[nadako]

Should work with this branch of the language server: https://github.com/nadako/haxe-languageserver/tree/wait_stdio

[nadako]

On a related note, I wonder if we should run our display tests in all possible flavors (simple cmdline, socket server, stdio server).

[SlavaRa]

@nadako Maybe you'll update the help in the compiler?