Soft parser update 2

[simerplaha]

• Towards Parser errors prevent CodeProvider execution (completion, goto definition etc) #104

Sorry for the large PR. I've attempted to implement most of the parsers, it helps in modeling the AST.

A notable change is that ASTs of type CodeAST (i.e. ASTs that store code tokens) also contain documentation: Option[Comments].

For example, in the following code, the multiline comments are stored within the function. Useful for displaying documentation on hover.

// Function documentation
// @param param A parameter of type `Param`
fn function(param: Param) -> ...

Test cases and documentation are pending. The following PRs will address them.

[tdroxler]

LGTM, reviewing every line of the parser is indeed a bit complicated, so I rather looked more the tests, which are very nice as usual.

[simerplaha]

Yep totally. Too hard to review at the moment, tests are needed.