Add keyword support to signature match

[vinistock]

Motivation

Step towards #2363

This PR continues the work on #2541 and adds support for matching keyword parameters.

Implementation

The logic becomes a bit more complicated once we start mixing positionals with keywords and splats of both types.

The important thing to keep in mind is that this is going to be used to guess which overload to show in signature help, which means we need to do partial matching. For example:

def foo(a, b); end

# We need to match all of these if we're going to present the signature
# to the user
foo
foo(1)
foo(1, 2)

Automated Tests

Added tests.

[andyw8]

Nice!