This removes unnecessary movn, movf, lds, and a few other instructions.
Any instructions that has not side effects and writes to a slot that
isn't used can be removed. A number of other optimizations can follow
from this:
- Implement the def-aliasing-var optimization better
- This function can be iterated as a fix point until no more
instructions are removed.
- If we implement slot renaming, then we no longer need to free slots
and can simplify the initial code generation a lot.
buffer/blit is difficult to use, and while buffer/push is the easiet
buffer manipulation function to use it only appends to the buffer.
buffer/push-at lets users manipulate buffers at any index - useful
for buffers used as an in-memory databases, for example.
