v0.3.0

Changelog:

• refactor: add "thread-safe" versions of certain functions
• perf(close): add translation step to simplify minfd/keep_fds specification
• perf(fditer): override FdIter::min() and Fditer::max() for efficiency
• ci: build for more supported targets
• feat(fditer): expose "possible" status of an FdIter
• fix: fix internal bugs
• refactor: major internal refactoring
• doc: improved documentation (and warnings)
• test: improved tests