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`
Cleaned up config.mk
Removed test from Makefile
Deleted test.c
Added initial bitmasking macros
iris: clean up some unused mnemonic code
Fixed arch documentation
Rearranged bits in ifthenelsemode

The reg1issigned field now comes after reg1 to simplify implementation
Replaced instruction structure with macros
Updated arch documentation with system call
Finished implementing instruction macros