Add tag decorator

[cdonovick]

Sorry, ignore this comment

[phanrahan]

LGTM. Thanks for the quick turnaround.

[phanrahan]

Shouldn't we also change encode ...

def encode(inst, *args):
    #print(inst, type(inst))
    if isinstance(inst, AbstractBitVector):
        word = int(inst) << inst.bitfield
    elif isinstance(inst,Enum):
        word = inst.value << inst.bitfield
    elif isinstance(inst,Product):
        word = 0
        for key in type(inst).field_dict.keys():
            word |= encode(getattr(inst, key))
    elif isinstance(inst,Sum):
        t = type(inst)
        if hasattr(t, 'tags'):
            i = t.tags[type(inst.value)]
        else:
            i = list(t.fields).index(type(inst.value))
        word = (i << t.bitfield) | encode(inst.value)
...

[cdonovick]

Probably, I don't use encode anywhere, feel free to modify as you see fit.

In the long term I think it would be an appropriate to remove encode and build anAssembler with the same logic. The AssembledADT infrastructure that is about to exist relies on people subclassing AbstractAssembler so that it can translate adt[key] and adt.attr to bv[idx]

[phanrahan]

Let me build out encode so that is assembles the pdp8 and riscv, and then we can figure out how to merge those ideas into Assembler.