This change begins to bind function bodies. This must be done as a
second pass over the AST, because dependencies between modules, and
even intra-module dependencies, might refer to top-level symbols like
types, variables, and functions, and so must be established first.
At the moment, the only node kind we handle is ast.Block, which
merely pushes and pops lexical scopes; however, the next step is to
implement the AST node-specific visitation logic for all statement
and expression nodes.
I've also rearranged how Scopes work to be a little easier to use.
The Scope type now remembers the **Scope slot in which it is rooted,
so that we can simply call Push and Pop on Scopes and have the right
thing happen.
c182d71c08