iris: start unparse.c to prettyprint instructions

* Have Makefile compile all *.c files into a single `iris` binary.
* New main() function for testing; should figure out how to refactor.
* Only fleshed out arithmetic instructions in unparse.c so far.
iris: off-by-one error resulting in infinite loop
iris: factor instruction mnemonics out of unparse.c
iris: unparse move instructions
iris: add function to look at bit representation of instructions
iris: clean up unparse's use of `error` in favor of always returning something
iris: minor clean up of unparse return values for arithmetic instructions
iris: unparse compare instructions
iris: add vim modelines to *.c
iris: have unparse functions take in `datum` instead of `ushort`
iris: clean up some unused mnemonic code
Merge branch 'master' into unparse

Conflicts:
	src/iris/Makefile
	src/iris/iris.c

* Modified the Makefile fairly heavily (yet still more to do)
* Moved testdecode.c -> tests/decode.c
* unparse.c, mnemonic.c, etc. will NOT compile yet (unmodified)
iris: move irissystem from sim.c to iris.c
iris: fix mnemonic.c
iris: fix unparse.c
iris: unparse normal jumps (yikes)
iris: unparse if/then/else jumps
iris: remove useless `break`s from mnemonic.c
iris: futz around with mnemonics; remove arrows from unparsed instructions
iris: remove unused error code I added earlier