Split up responsibilities of Compiler

[fwcd]

For improved testability, it would be cool to factor out the explicit path and file handling from Compiler and divide up its responsibilities into individual methods or perhaps even different modules:

• Generate instructions as Vec<Instruction>
• Prettyprint Vec<Instruction> to a .asm source file (or, if that's not sufficient, introduce a new struct for representing a complete .asm file abstractly and generate that instead)
• Running nasm on the resulting .asm source file
• Linking the object files

This should also make it easier to perform non-black box testing, since we could perform assertions directly on the generated instructions for a block of code.