GH-101291: Rearrange the size bits in PyLongObject

[markshannon]

This PR rearranges the bits in what was ob_size, to slightly speedup the most common operations and to prepare for storing the tagged 2-complement value directly in a future PR.

The new layout is as follows:

• Bits 0 and 1: 1 - sign. I.e. 0 for positive numbers, 1 for zero and 2 for negative numbers.
• Bit 2 reserved (probably for the immortal bit)
• Bits 3+ the unsigned size.

The bulk of the change is removing all the uses of Py_SIZE and Py_SETSIZE, and replacing them with a new set of inline functions.
It disturbs me how much we use unchecked casts, but that's a separate issue...

This will, inevitably, break Cython generated code again.

Performance measurement shows no significant change: https://github.com/faster-cpython/benchmarking/tree/main/results/bm-20230302-3.12.0a5%2B-ce6bfb2

• Issue: Restore (or beat) Python 2 performance for arithmetic operations on ints that fit into a single word #101291