Rename all occurences of AST (#413)
* ast.{cc,h} => ir.{cc,h}
* binary-reader-ast.{cc,h} => binary-reader-ir.{cc,h}
* ast-writer.{cc,h} => wat-writer.{cc,h}
Everything else changes ast => wast.
When deciding between wast vs. wat, the wat format should only allow for
a single module, without assertions.
