Language service crate

[minestarks]

This adds a language_service Rust crate. The language service is simply a struct that maintains a compilation state for each known document. It receives document edits and updates the current compilation state based on those edits. It provides methods (get_completions, get_definition, hover) to get information about the current compilation.

Review note This PR is a subset of the draft PR at #364 . That PR is an end-to-end implementation with everything wired up and functional in the playground and VS Code. You may want to refer to #364 if you have questions about how some of the things defined in this PR will end up getting used.

Review note the actual editor feature implementations at completion.rs, definition.rs and hover.rs are meant to be placeholders with very limited functionality (see the unit tests for the very few cases we do support). Fleshing out the actual features will be done in later PRs.

[swernli]

Since this introduces a new top-level folder, could you also add it to the code owners file? Perhaps with you, Bill, and Ian as owners?

[minestarks]

Addressed @idavis and @SamarSha 's comments.

@SamarSha - I had to update the call sites for SourceMap methods, which subtly changes the assumptions at the call site in some cases. A careful look would be appreciated to make sure I didn't break any behavior.

[bamarsha]

The changes related to SourceMap look good.