Syntactic sugar for declaring signatures

[mpharrigan]

A traditional function signature gives is an ordered mapping of parameter name to its data type. In Qualtran, we have both inputs and outputs as part of the function signature with many "registers" doing double-duty as both inputs and outputs ("THRU" registers). We support classical ndarrays of variables with the shape parameter. Suggest supporting building signatures with a syntax like:

Signature.build(ctrl=1, ints=QInt(8)[5])
Signature.build(x=(QAny(3), QBit()[3]))

for

Signature([Register('ctrl', QBit()), Register('ints', QInt(8), shape=(5,))])
Signature([Register('x', QAny(3), side=Side.LEFT), Register('x', QBit(), shape=(3,), side=Side.RIGHT)])

respectively