Optionally use CORE-MATH correctly rounded elementary functions

[ridgeworks]

Under control of CMake option USE_CRMATH (default=OFF), build a local copy of library containing a subset of CORE-MATH functions from repo https://github.com/indigobio/core-math-binary64.

If option is OFF, use existing explicitly rounded functions currently in pl-arith.c. (under control of O_ROUNDING_UP_DOWN).

Caveats:

1. Only tested on MacOS, X86 architecture.
2. compile flags for local crmath library set to "-O3 -ffp-contract=on -fno-math-errno" as per external repo. Is this right for all architectures/compilers. (Note: no support for different FP rounding in WASM).
3. Limited CMake experience so additions should be reviewed.

[JanWielemaker]

Merged after some refactoring. Notably reuse the external dependency with as little as possible own code and pin the version by git HASH for security and reproducibility purposes.