Exports now have a dirty flag and are rewritten on flush if this flag
has been set.
A couple other minor changes have been made based on Andrew's review.
Thus far, we only generate the type, function, export, and code
sections. These are sufficient to generate and export simple functions.
Codegen is currently hardcoded to `i32.const 42`, the main goal of this
commit is to create infrastructure for the container format which will
work with incremental compilation.