thunk: introduce thunk_t.free and thunk_free()

While it was convenient to support bare libc free()ing of thunks
wherever appropriate, it prevented any form of caching thunk
encapsulating environments.

This commit introduces thunk_free() for freeing thunk_t
instances, and adds a .free() member to the thunk for registering
a thunk-specific free method.

For plain thunk instantiating, where the environment size is
known and fixed for a given thunk, a very simple free list has
been added.  This added a free_next pointer to the thunk's
environment struct, and a thunk-specific free method putting the
instance's environment back on the thunk-specific free list.  No
shrinking is ever done currently, and there's never cleanup of
any of this, it's assumed these will be relatively bounded,
leveling off at a high watermark, and simply abandoned at process
exit for now.

For split allocated-initialized thunk instances, no caching is
attempted for now.  This could be trivially improved at least for
non-payload-utilizing cases, as these would be the same size as
the plain instantiated thunks so they could just use the same
free lists.  For the payload-utilizing cases it'd be more tricky,
in the interests of simplicity for now, all the split cases just
use malloc()+free() as before, but via the thunk_t.free() method.

This commit should already significantly reduce the amount of
malloc() and free() in most cases.

0 files changed, 0 insertions, 0 deletions