Add API for unbiased "random numbers"

[stevew817]

There exists a footgun in using raw RNG output when in reality you need a number between x and y with evenly distributed probability. This is called "modulo bias", and a good explanation can be found e.g. over at Kudelski.

Should the PSA Crypto API, which already provides a function to get whole bytes with uniform distribution, be extended with an API that gives an unbiased uniformly-distributed number between x and y?