Move QASM 2 printing into its own module

[danielgaskins]

Summary of changes

Moves Qasm2Printer and branch normalization into modules/qasm2_printer.py, leaving qasm2.py at 230 lines. Validation and printing still share the same normalization; the module now calls a named validation function.

Normalized branches have named fields instead of a tuple. The printer reads register sizes from the program, and its existing explicit-size arguments still work.

Closes #447.

All 1,006 tests pass with three existing skips. Before/after comparisons match exactly for serialized branches, validation errors and transformation results. Black, isort, Pylint, mypy, license headers and the Sphinx build with warnings treated as errors all pass.

Fresh wheel and source-distribution installs each pass the same 1,006-test suite (three existing skips).

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