Refactor PVF preparation memory stats (#6693)

* Refactor PVF preparation memory stats

The original purpose of this change was to gate metrics that are unsupported by
some systems behind conditional compilation directives (#[cfg]); see
https://github.com/paritytech/polkadot/pull/6675#discussion_r1099996209.

Then I started doing some random cleanups and simplifications and got a bit
carried away. 🙈 The code should be overall tidier than before.

Changes:
- Don't register unsupported metrics (e.g. `max_rss` on non-Linux systems)
- Introduce `PrepareStats` struct as an abstraction over the `Ok` values of
  `PrepareResult`. It is cleaner, and can be easily modified in the future.
- Other small changes

* Minor fixes to comments

* Fix compile errors

* Try to fix some Linux errors

* Mep

* Fix candidate-validation tests

* Update docstring