AST changes for function and type bindings

[j14159]

@danabr raised a point about type definitions vs types themselves in PR #116 , e.g.

type opt 'a = Some 'a | None
type int_opt = opt int

The name in the left-hand side of each of these could be viewed in a similar manner as function names in let bindings, that is, independent of their variables and members/bodies whereas opt int is a concrete type. Similarly as raised by @ypaq and others elsewhere:

let f x = x + x

could be viewed as syntactic sugar for

let f = fun x -> x + x

or something to that effect.

We might then add an alpaca_type_def node that binds variables and members to a type name while a member of a type binding stays as an alpaca_type AST node since it is in fact a concrete type. Functions then might be decomposed into two things as well:

• an alpaca_fun node that lists function versions, each of which has their associated variables and bodies.
• alpaca_fun_def that binds a name and arity (the latter for convenience) to a single alpaca_fun AST node.

This should make lambdas fairly simple to implement and makes types, functions, and values all operate in a similar manner.

Thoughts?