Add the target-independent trans module

Add the target-independent trans module

This handles translation of the AST to an SSA form. Currently not doing
any control flow constructs inside a function, so functions just have a
single basic block with linear SSA. This does some basic load forwarding
and de-duplication optimizations since they're straightforward.

The most important effect here is translating the globally resolved
names into offsets into the closure record, and listing the variables
that need to be copied into constructed closures.