Patch mostly by Serhiy Storchaka.

I'm checking this in without a test because not much of this code
is tested and I don't have time to work up the necessary extensions
to the existing test framework.

The patch itself was tested by the person who reported the bug.

mmap obmalloc arenas so that they may be immediately returned to the system when unused (closes #20494)

The '**' operator is available only since 8.5 and in any case such large
integers are not supported on Tcl <8.5.

Issue #20426: When passing the re.DEBUG flag, re.compile() displays the debug output every time it is called, regardless of the compilation cache.

unicode strings only).  Improved error handling in variables-related commands.

exprlong() and exprboolean().

from the old green snake to the new new blue and yellow snakes.

introduced with the issue19081 tests.

Py_VerboseFlag print that can never be triggered.

prefix[0] is always equal to 0 at this point in the code.

test_ns dict instead of the actual globals() and locals().  Suggested
after review by Thomas Wouters.

is present.