Integrate into Cython test suite later

Rename the options of the "check_size" feature to make them more obvious: "warn" warns, "error" fails, and "extend" silently allows extending.
Closes #2627.

Enable "check_size=False" for the NumPy ndarray and dtype extension types to avoid warning about otherwise backwards compatible changes.

Add new "language_level=3str"

 ENH: add check_size option to ctypedef class for external classes 

Remove the new 'str_is_str' directive again and replace it by a new "language_level=3str" that resembles "language_level=3" but keeps unprefixed string literals and the 'str' builtin type unchanged.
See #2565.

Closes #2632.

Ignore the "c_string_encoding" directive when looking for a "coding" cookie in the first two source lines.
Closes #2632.

DOC, TST: test and document external extension type attribute aliasing

MAINT: if subtree test fails, print out all stdout, stderr not just last

Add a new directive 'str_is_str=True'

Add a new directive 'str_is_str=True' that keeps unprefixed string literals and the 'str' builtin type unchanged even when 'language_level=3' is enabled.
See #2565.

TEST: add test for 'binary incompatibility' warning

Deduplicate constant tuples and slices to generate them only once and share them across the entire module.
Closes #2292.