Make FastRandomContext support standard C++11 RNG interface

Summary:
This makes it possible to plug it into the various standard C++11 random
distribution algorithms and other functions like std::shuffle.

Tis is a backport of Core PR12742

Test Plan:

make check

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3413