Add a new public crate function `cross_compile_from_to()` using
`target_lexicon::Triple` arguments instead of plain strings
used in `cross_compile()`.
Deprecate `pyo3_build_config::cross_compile()` since v0.17.
Attempt to extract common code patterns into methods and standalone
helper functions. Add docstrings to the new private items.
Make some of the new helper functions public within the PyO3 crate
and reuse them in the build scripts.
Add PYO3_CROSS_PYTHON_VERSION parsing unit test.
Add a ChangeLog entry mentioning the new `pyo3-build-config` API.
David Hewitt