Signature.build with flexible typing and shape syntax

[mpharrigan]

• a new ShapedQCDType capable of specifying quantum arrays natively through subscripting (e.g., QInt(8)[20])
• upgraded Signature.build syntactic sugar. Signature.build can now seamlessly handle fully instantiated types, shaped types, positional lists of registers/signatures, and granular side directives via tuples. fixes Syntactic sugar for declaring signatures #1769

            >>> from qualtran import QBit, QUInt
            >>> sig = Signature.build(ctrl=QBit()[5, 5], system=QUInt(32))
            >>> sig == Signature([
            ...     Register('ctrl', QBit(), shape=(5, 5)),
            ...     Register('system', QUInt(32))
            ... ])
            True

            >>> sig = Signature.build(a=(QBit(), QBit()), b=(None, QBit()))
            >>> sig == Signature([
            ...     Register('a', QBit(), side=Side.THRU),
            ...     Register('b', QBit(), side=Side.RIGHT)
            ... ])
            True

[gemini-code-assist]

Code Review

This pull request introduces a more flexible syntax for building Signature objects by adding ShapedQCDType and enhancing Signature.build. The changes allow for more intuitive definitions of registers, including shaped arrays and side specifications. My review found one issue where the implementation of Signature.build for positional arguments is more restrictive than its documentation suggests, only handling lists instead of general iterables. I've suggested a fix to align the implementation with the documented behavior.