Extract ISA and assembler to a separate repo/project

[mortbopet]

Depends on #301, #303, #306

The ISA definition and assembler/disassembler library in Ripes isn't inherently tied to Ripes. Rather, it represents a neat little library for defining relevant information of an ISA, and then based on this, automatically generate assemblers and disassemblers from said information.

Back when I initially wrote the assembler/disassembler for Ripes i looked around for any good opensource alternatives, of which (back then) i found none. Everything was either too ISA specific or sauced into some larger context.
The benefit of the Ripes assembler/disassembler/isa infrastructure is that it's generic - meaning that it should ideally work with any instruction set. Second, it's (relatively speaking) tiny compared to e.g. using LLVM. As such, I think it could be a great little open-source project that other people could use if they quickly wanted to generate an assembler/disassembler for their own custom ISAs.