mmap-backed allocator

[program--]

This PR adds a utility allocator class that provides allocation over mmap'd files. In theory, this would be useful for containers that store fixed-size amounts of data (i.e. mdarray), without consuming the same amount of RAM.

Additions

• Adds ngen::mmap_allocator<T, PoolPolicy>, which implements a STL-compatible allocator. PoolPolicy provides a policy approach for default storage locations, and potentially other things (still need to decide if it is needed...). Has a runtime-customizable backing directory.

• Adds ngen::basic_pool, which is a filesystem pool using C's open and close. Currently defaults to the system's tmpfs.

Testing

• All unit tests pass locally with GCC 13, clang 16, and IntelLLVM 2023.
• Manual testing with GCC 8 + GCC 9 using godbolt:
  • GCC 8.1: https://godbolt.org/z/8rP8cs7Mo
  • GCC 9.1: https://godbolt.org/z/q6Pvqv8Td

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

Target Environment support

• Linux
• macOS

[program--]

We may be able to close this PR actually, Boost.Interprocess provides memory mapping-based allocation, so its probably favorable to use that instead.