Standard library documentation

[liquidev]

Mica needs some canonical way of documenting the standard library (and other modules.) To facilitate this, a documentation comment construct should be added into the language.

#! This is module-level documentation.

# This is a normal comment

## This is a documentation comment documenting an item
struct Example impl
    ## This is another doc comment, but documenting a function.
    func example() = nil
end

The Rust API should expose methods that allow for extracting these doc comments from the source code, such that tooling for building autogenerated docs can be built.