Add functions to generate random numbers in a specified range.

Add functions to generate random numbers in a specified range.

This adds 3 new variants of the random() function:

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

Each returns a random number x in the range min <= x <= max.

For the numeric function, the number of digits after the decimal point
is equal to the number of digits that "min" or "max" has after the
decimal point, whichever has more.

The main entry points for these functions are in a new C source file.
The existing random(), random_normal(), and setseed() functions are
moved there too, so that they can all share the same PRNG state, which
is kept private to that file.

Dean Rasheed, reviewed by Jian He, David Zhang, Aleksander Alekseev,
and Tomas Vondra.

Discussion: https://postgr.es/m/CAEZATCV89Vxuq93xQdmc0t-0Y2zeeNQTdsjbmV7dyFBPykbV4Q@mail.gmail.com