Refactor functions implementation

Previously, functions were implemented with a separate set of types that
had their own code paths in the compiler, eval, etc. These changes
refactor the function implementation so that functions are implemented
as rules with one or more arguments.

By representing functions as rules, we can avoid special casing required
to support functions, e.g., during parse and compile there are a number
of steps that required special casing for functions:

- Parser needed separate grammar definitions for functions (which
  prevented them from being chained or using else)

- Compiler needed separate resolver and type checker implementations
  which was a source of bugs.

In some cases, special casing is unavoidable for now (e.g., during eval)
however this could be improved in the future.

Fixes #471
Fixes #467
Fixes #463