📚 Add documentation to function types

[timfennis]

Parses documentation from functions defined in rust and adds the to the runtime.

Also adds a docs function to return the documentation as a string:

λ trim_start.docs
fn(String) -> Removes leading whitespace from the string.
fn(String, String) -> Removes leading occurrences of `pat` from the string.

Todo

• Add names of parameters to the signature

[timfennis]

There are so many more improvement but it's time to merge...