Closes https://github.com/cython/cython/issues/4655

Add a "minimal compile" build mode that speeds up the compiler somewhat but keeps the number of compiled modules low.

runtests: Print shard number also for each command in the EndToEnd test error output to make it easy to find in the logs.

runtests: print the shard number in front of the test name to show which was the last test in case it hangs.

See https://github.com/cython/cython/pull/4583#issuecomment-1036452181

Fixes https://github.com/cython/cython/issues/4092

Nested cdef classes are not tested because they don't officially work
at this stage (see https://github.com/cython/cython/issues/4572).
It should be tested if/when they're fully supported.

Limited API: Fix line number reporting in tracebacks that accidentally used 'c_line' instead of 'py_line'. In fact, we cannot currently show the C line, so disable C line reporting completely here.

Follow PEP-484 and warn about "func(x: list = None)" without an explicit "typing.Optional[]" declaration.

See https://github.com/cython/cython/issues/3883

Avoid an unnecessary inefficiency in a regex, although it is not relevant in practice since the regex is only applied to Cython's only C code.
Found by GitHub CodeQL.

Rewrite the section that describes the differences between __cinit__() and __init__(), to make it clearer when to use which.

Partially reverts https://github.com/cython/cython/commit/1027db04fb7c5bbfea2b9bcdfce0b1f384e26a93

Make the extension types tutorial actually understandable by itself, without referring to obscure "previous code" and "our integration example" that come out of nowhere.

Allow globally forcing C file regeneration by setting the env var CYTHON_FORCE_REGEN=1, e.g. from external build systems.