[open-systems] QCDType, CDType, QDType

[mpharrigan]

This pull request is the first in a series for supporting open system (classical data & measurement), roadmap item #445.

QCDType

• The existing QDType base class for quantum data types now descends from the more general QCDType. CDType is introduced to represent classical data types.
• CBit is added, which is a single classical bit (analogous to QBit).
• Python type annotations are updated as needed. In particular, Register's dtype argument can be either quantum or classical.
• This existing, runtime, Qualtran-based type checking basically works without modification. It won't let you plug a quantum wire into a classical bloq (or vice versa)

_ControlledBase

• I've refactored the class hierarchy of Controlled. The circuit-implementation-agnostic bit is now in _ControlledBase from which Controlled (which uses a "total control" circuit) and ControlledViaAnd descend.
• CtrlSpec supports classical data types (CDType)
• The logic for choosing the correct default controlled meta-bloq based on control spec is factored out into a function
• This is to support plugging in a new, forthcoming ClassicallyControlled metabloq

Drawing

• The musical score diagrams draw classical wires in blue.

Documentation

• The existing QDType system was not really documented (at least not with its own notebook). I went ahead and wrote docs for the data type system.
• There is a section at the end about the QCDType class hierarchy

Naming

• Before this PR and throughout the framework, qdtype: QDType and dtype: QDType have been used interchangeably. After this PR, idiomatically: qdtype should be the variable name used for a QDType and dtype should be reserved for variables that can be either quantum or classical (i.e. QCDType). Where the variable name isn't backwards-compatibility breaking, I've made those changes in this PR; but there are many places where dtype is used where qdtype would be more appropriate
• Before this PR and throughout the framework, bitsize and num_qubits (as variable/method names) have been used interchangeably. This PR adds methods/properties that aim to disambiguate these quantities. bitsize or num_bits is the total number of quantum and/or classical bits. num_qubits is strictly qubits (resource counting still counts strictly qubits). num_cbits is added; which is the number of strictly classical qubits.

Non-changes (yet)

• While CDType, CBit, and CtrlSpec(CBit()) are introduced, there's no real way to use them (with the exception of Cast, which is likely not what you really want).