- Introduce new `Path` type and an array of build paths on the build context.
- Resolve input and output paths/files early (before parsing).
- Error early if inputs are missing or outputs are directories.
- Plumb new file path generation into linker stage instead of its adhoc method.
TODO:
- Remove more adhoc file path generation in parser and linker stage.
- Make intermediate object file generation use new path system.
- Round out and robustify Path helper functions.
`_DIGIT_NAILS` is defined as 4, meaning that we use 60 out of every 64 bits.
We can use as few as 1 nail, using 63 bits out of every 64, and all tests will still pass.
However, it needs more testing to see if that's a worthwhile change to make.
For the tests to work properly when changing the nails, Python needs to know about it as well.
In addition, compile the big math code with `-o:speed` going forward.
