fix broken link in readme

refactor file parsing, tests

This dramatically refactors parsing files into txtar-archives, splitting
the complex process-lines function into smaller helpers.

It issues the same number of allocations, and the performance remains
largely unchanged.

In the process of this refactor, we:

- Fix edge-case bugs with empty files and preserving whitespace.
- Adopt a "%" prefix to indicate a type or function is internal.
- Organize the functions into sections and add helpful comments.
- Tie the test groups directly to the exported library functions.

refactor txtar parser to use record-based state management

- Replace list-based state with parser-state record type
- Add more precise type annotations in exports
- Remove redundant string-prefix? implementation
- Import string-index from SRFI-13

adjust api to be more consistent, scheme-like

fix spacing

remove trailing spaces

add license blurb to readme

adjust spacing

update readme

fix bugs, expand tests, utilize srfi libraries

initial commit