Move insertion resize logic into set_insert_key().

Move insertion resize logic into set_insert_key().

Simplifies the code a little bit and does the resize check
only when a new key is added (giving a small speed up in
the case where the key already exists).

Fixes possible bug in set_merge() where the set_insert_key()
call relies on a big resize at the start to make enough room
for the keys but is vulnerable to a comparision callback that
could cause the table to shrink in the middle of the merge.

Also, changed the resize threshold from two-thirds of the
mask+1 to just two-thirds.  The plus one offset gave no
real benefit (afterall, the two-thirds mark is just a
heuristic and isn't a precise cut-off).