Added all I-format instructions for the RV32I set

Some things to note:

SLLI, SRLI, and SRAI should get their own format since the immediate
field length is smaller.  Right now, you can give them a 12-bit
immediate constant and it'll assemble it just like any other I-format
instruction.

CSR* instructions are all I-format instructions as well.  But, I'll wait
to implement these until after I study the ANGEL sources in greater
detail to get the complete set of CSR* instruction encodings.