Re: Optimization of some jsonb functions

I pushed the first few parts.  The attached is a rebased copy of the
last remaining piece.  However, I didn't quite understand what this was
doing, so I refrained from pushing.  I think there are two patches here:
one that adapts the API of findJsonbValueFromContainer and
getIthJsonbValueFromContainer to take the output result pointer as an
argument, allowing to save palloc cycles just like the newly added
getKeyJsonValueFromContainer(); and the other changes JsonbDeepContains
so that it uses a new function (which is a function with a weird API
that would be extracted from findJsonbValueFromContainer).

Also, the current patch just passes NULL into the routines from
jsonpath_exec.c but I think it would be useful to pass pointers into
stack-allocated result structs instead, at least in getJsonPathVariable.

Since the majority of this patchset got pushed, I'll leave this for
Nikita to handle for the next commitfest if he wants to, and mark this
CF entry as committed.

Thanks!

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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