Add ASM builder

[Horrowind]

trafficstars

Since you are adding new items to the asar wishlist:

How about a ASM builder API (similar to LLVM)? The idea would be that if I want to create ASM with a tool, I wouldn't need to write it to a string buffer to be read again, but instead I could write something like this:

asm_builder_label(asm, "labelname");
asm_builder_lda(asm, 0x1234);
[...]
asm_builder_bne(asm, "labelname");
asm_build(asm);

Clearly this is far fetched, will need additional thought and might as well cause a rewrite of the core of asar. I would hope that this would make the semantics of asar precise, since many of the commands in asar would map directly to those function calls.