reduce size of fmgr_builtins array

Hi all,

Currently, we include the function name string in each FmgrBuiltin
struct, whose size is 24 bytes on 64 bit platforms. As far as I can
tell, the name is usually unused, so the attached (WIP, untested)
patch stores it separately, reducing this struct to 16 bytes.

We can go one step further and allocate the names as a single
character string, reducing the binary size. It doesn't help much to
store offsets, since there are ~40k characters, requiring 32-bit
offsets. If we instead compute the offset on the fly from stored name
lengths, we can use 8-bit values, saving 19kB of space in the binary
over using string pointers.

--
John Naylor https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services