Implement function, lexical scope, and let

[alandipert]

Background

eval in main.c knows how to find and execute special forms - functions defined in C and referred to the global environment - but user functions, the let binding, and lexical scope still all need to be implemented. The bottom of functions might be let, or vice-versa - they are in essence two ways to create a closure, and should share semantics.

Implementing implies a mechanism for merging scopes and supporting the idea of a scope chain, which is also not done. Before we do this, we probably want to move away from using alists for environments, and use real hash tables instead.