Functions to return random numbers in a given range

Attached is a patch that adds 3 SQL-callable functions to return
random integer/numeric values chosen uniformly from a given range:

random(min int, max int) returns int
random(min bigint, max bigint) returns bigint
random(min numeric, max numeric) returns numeric

The return value is in the range [min, max], and in the numeric case,
the result scale equals Max(scale(min), scale(max)), so it can be used
to generate large random integers, as well as decimals.

The goal is to provide simple, easy-to-use functions that operate
correctly over arbitrary ranges, which is trickier than it might seem
using the existing random() function. The main advantages are:

1. Support for arbitrary bounds (provided that max >= min). A SQL or
PL/pgSQL implementation based on the existing random() function can
suffer from integer overflow if the difference max-min is too large.

2. Uniform results over the full range. It's easy to overlook the fact
that in a naive implementation doing something like
"((max-min)*random()+min)::int", the endpoint values will be half as
likely as any other value, since casting to integer rounds to nearest.

3. Makes better use of the underlying PRNG, not limited to the 52-bits
of double precision values.

4. Simpler and more efficient generation of random numeric values.
This is something I have commonly wanted in the past, and have usually
resorted to hacks involving multiple calls to random() to build
strings of digits, which is horribly slow, and messy.

The implementation moves the existing random functions to a new source
file, so the new functions all share a common PRNG state with the
existing random functions, and that state is kept private to that
file.

Regards,
Dean