Support nested function definitions

[OceanOak]

This PR adds support for nested function definitions.
You can now define a helper function inside a function body, instead of only at the package top level:

let reverse (list: List<'a>) : List<'a> =
  let reverseHelper (list: List<'a>) (acc: List<'a>) : List<'a> =
    match list with
    | [] -> acc
    | head :: tail -> reverseHelper tail (push acc head)

  reverseHelper list []

The let f (x: T) : R = ... form is now a variant of the regular let expression, so it works anywhere a let can appear (function bodies, match arms, lambda bodies, etc.).

How it works

• Desugars to a let-bound lambda; type annotations are parsed but dropped at runtime (lambdas are untyped)
• Self-recursion is supported - a nested fn can call itself by name

Limitations

• A nested fn whose name collides with a fn/value already in scope is rejected
• Mutual recursion between nested fns isn't supported - calling a nested fn defined later gives a "forward calls between nested functions aren't supported" error