ta: remove seperate internal "ext" header

The ta_ext_header was allocated on demand for allocations which have
child-allocations or destructors. In theory, it saved 2 words for every
TA leaf allocation. It had the very API-visible problem that setting a
parent or destructor could fail. (Although in most cases, the failure
was part of an allocation call anyway. Also, mpv code generally used the
early-failure variants, so it didn't matter.)

I think this was a bit too complex. These 2 words don't really matter;
if you have memory allocations where you are worried about overhead,
then these simply shouldn't use TA. Also, we never added new features to
TA that would have needed more "optional" header fields, which would
have justified the use of such a separately allocated header struct.

This uses quite straight-forward data structures. The only strange thing
is that ta_header.parent is NULL for most child allocations. That is
because we don't want to iterate over all children when the parent is
reallocated (yes, that is allowed, yes mpv makes use of it).

The new code has a few more special cases, because the list sentinel
concept isn't used anymore. Using it would have made the code more
unnatural/complex, because ta_ext_header doesn't exist anymore.

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