LSP server, initial framework

[digama0]

Currently it doesn't actually do anything, but this implements:

• a --lsp flag to start the server
• a multithreaded backend which dispatches request jobs
• parsing for the LSP protocol for a few commands:
  • hovers
  • go to definition
  • $/eval, a custom LSP extension which will be used to run SML code from the client

[mn200]

Neat!