numeric input changes

The attached patch changes the rules for input to
int2/int4/int8/float4/float8 to consistently allow leading and
trailing whitespace in the input. This is for conformance with the
SQL standard, as discussed in an earlier -patches thread.

I also added a bunch more regression tests for the input format
accepted by these types.

I noticed one bug, or at least instance of bit rot: the float8 code
attempts to parse "Infinity" and "-Infinity" specially. Unfortunately,
this doesn't actually work:

nconway=# select 'Infinity'::float8;
ERROR: type "double precision" value out of range: overflow
nconway=# select '-Infinity'::float8;
ERROR: type "double precision" value out of range: overflow

I'm inclined to just remove this attempted functionality from the
code, rather than fix it to work properly. Any comments?

I intend to apply this patch tomorrow.

-Neil