GH-101362: Optimise pathlib by deferring path normalisation

[barneygale]

PurePath now normalises and splits paths only when necessary, e.g. when .name or .parent is accessed. The result is cached. This speeds up path object construction by around 4x.

PurePath.__fspath__() now returns an unnormalised path, which should be transparent to filesystem APIs (else pathlib's normalisation is broken!). This extends the earlier performance improvement to most impure Path methods, and also speeds up p.joinpath('bar') and p / 'bar'. edit: will fix separately.

This also fixes GH-76846 and GH-85281 by unifying path constructors and adding an __init__() method. edit: will fix separately.

• Issue: Optimize pathlib path construction #101362