Add numeric <-> int8 and numeric <-> int2 conversion functions, as well

Add numeric <-> int8 and numeric <-> int2 conversion functions, as well
as a unary minus operator for numeric.  Now that long numeric constants
will get converted to NUMERIC in early parsing, it's essential to have
numeric->int8 conversion to avoid 'can't convert' errors on undecorated
int8 constants.  Threw in the rest for completeness while I was in the
area.
I did not force an initdb for this, since the system will still run
without the new pg_proc/pg_operator entries.  Possibly I should've.