Add pg_strtoupper and pg_strtolower functions

Hi,

I came across pg_toupper and pg_tolower functions, converting a single
character, are being used in loops to convert an entire
null-terminated string. The cost of calling these character-based
conversion functions (even though small) can be avoided if we have two
new functions pg_strtoupper and pg_strtolower.

Attaching a patch with these new two functions and their usage in most
of the possible places in the code.

Thoughts?

Regards,
Bharath Rupireddy.