Give _PyInstructionSequence a python interface and use it in compiler tests

[iritkatriel]

Expose _PyInstructionSequence as a PyObject, and then the compiler tests would be able to work with it directly.

By giving this object functions to add a label/instruction, the logic of translating a convenient python representation to a C sequence does not need to be duplicated in the test harness, the process of constructing an instruction sequence from tests becomes the same as the one used in codegen, and re-uses the implementation of the instruction sequence data structure.

Linked PRs

• gh-117680: make _PyInstructionSequence a PyObject and use it in tests #117629
• gh-117680: Fix msvc warning in instruction_sequence.c #118326

[terryjreedy]

/refuses/re uses/? (last line)