cpp/experimental/lib extensions 2

Version 2 of the C++ Extensions for Library Fundamentals, ISO/IEC TS 19568:2017, defines the following new components for the C++ standard library, in addition to those defined in version 1:

Simple random number generation facilities
A per-thread engine of type std, initialized to an unpredictable state, is maintained for each thread and used by the functions below.

Merged into C++17
The following components of the library fundamentals v2 have been adopted into the C++17 standard.

Merged into C++20
The following components of the library fundamentals v2 have been adopted into the C++20 standard.