Clean up jsonb code.

The main target of this cleanup is the convertJsonb() function, but I also
touched a lot of other things that I spotted into in the process.

The new convertToJsonb() function uses an output buffer that's resized on
demand, so the code to estimate of the size of JsonbValue is removed.

The on-disk format was not changed, even though I refactored the structs
used to handle it. The term "superheader" is replaced with "container".

The jsonb_exists_any and jsonb_exists_all functions no longer sort the input
array. That was a premature optimization, the idea being that if there are
duplicates in the input array, you only need to check them once. Also,
sorting the array saves some effort in the binary search used to find a key
within an object. But there were drawbacks too: the sorting and
deduplicating obviously isn't free, and in the typical case there are no
duplicates to remove, and the gain in the binary search was minimal. Remove
all that, which makes the code simpler too.

This includes a bug-fix; the total length of the elements in a jsonb array
or object mustn't exceed 2^28. That is now checked.