This lays the groundwork for other on-the-fly optimisations,
like passing literals through in order to do const folding
in linear time, while compiling.
* Implement basics.
* Execute code
* Add wasm2wat test cases.
* abi_loc_for_arg for stack.
* Assert that sp_depth is 0 at the epilogue
* Do 32bit add.
* Assert that RAX can be used as a scratch register
* Reuse assembler.
* Align stack slots.
