Parse RBS overloads into signatures

[andyw8]

Motivation

In this PR we are beginning fuller indexing of RBS, by handline multiple signatures per method.

Implementation

Previously, arguments was a field of Entry::Method. Now an Entry::Method has a signatures field (of type array), and each signature has the array of arguments.

Not covered yet:

• Return types
• Individual block arguments

Automated Tests

The tests are mostly using a selection of methods from Ruby core to verify the various forms of signatures in use.

Manual Tests

There is no integration with the LSP features yet so this cannot be easily manually tested.

[andyw8]

(have moved the Ruby LSP changes out to andyw8/support-multiple-signatures)

[andyw8]

@soutaro this is still WIP, but thought you may be interested to see our approach.

[andyw8]

There's one thing I need to fix but I'd like to start getting reviews on this.