Add an assembler

[jrmuizel]

This should be able to take the output of the disassembler and assemble it into equivalent byte code.

[fkaa]

I've been tinkering with this a bit and haven't really arrived at a solution yet.

On the one hand it's really nice to have lightweight parsing/disassembling by just storing the offsets/slice of the operands and then having helper methods to get the operand info. However I haven't found a way to make it ergonomic when assembling.

The "easy" way I believe would be to just have a (relatively) fat Instruction struct which contains the full opcode and operands. There are a few instructions which don't fit into this though; custom data (defining a constant buffer inline with variable size) and most dcl_ instructions have custom operands (sometimes just plain u32 rather than operand).

Edit: re-reading your comment I'm not sure if you're talking about assembling the stdout output of the disassembler or the struct you get in consume_instruction