Split up datatype definition modules (#1835)

Long overdue.

- prior releases included the `qualtran.dtype.XXX` aliases for QCDTypes
(the data type classes and objects used in your quantum programs)
- This PR moves the source of truth to private submodules in this
package and makes the `qualtran._infra.dtype` imports into aliases
- Most people are probably using the `qualtran.XXX` re-exports anyways,
which remain unchanged.

This is just moving code. The only "change" is moving the assert_to_bits
test helper is now in the public API.

Author: mpharrigan

qualtran/__init__.py

@@ -53,7 +53,7 @@
     QVarT,
 )
 
-from ._infra.data_types import (
+from .dtype import (
     QCDType,
     CDType,
     QDType,