Allow writing assembler format and have it auto-converted into DR IR

I thought we already had a long-standing feature request for this but I can't find it. The idea is to allow the tool writer, when dynamically generating sequences of instructions, to write them in conventional assembler format and have a layer that converts them into DR IR. While the INSTR_CREATE_ macros try to look like assembly, they are still bulky and have a lot of boilerplate.