C++ | newCache v2

[JATothrim]

Hello!

I have some upgrades to the newCache + project code:

• libmapped_file: memory mapped file I/O library (see first commit) This is my own library in making, partly unrelated to opencubes project. OpenCubes just happens to be first client for the library outside my projects. The library source files have explicit MIT license prepended to them.
• Upgraded CacheReader with libmapped_file
• New CacheWriter that writes out the hashy in parallel
• Dropped old cache code.
• Reduced memory use for Cube struct (was 16-bytes, now 8-bytes)
• Refactored cubes.cpp thread scheduling.

Overall I can do now N=13 in less than 310 seconds give or take. :smile: There still is no any kind of compression of the PolyCube data...

To generate starting cache files run cubes -n 10 -t <threads> -w -s followed by cubes -n 11 -t <threads> -w -s -u to continue with split cache files.

EDIT: I have re-arranged my repository and enabled GPG verified commits. The default branch https://github.com/JATothrim/opencubes is now fork of mikepound/main and includes the changes in this PR. @bertie2 can you take a look on this?