A useful source reader implementation to pass in a source saved as an
in-memory string into the tokenizer.
This replaces the string_src_reader in the tokenizer_test and is even a bit
more flexible, by allowing any aplf_string_view as the source.
- Simplify return types: Many functions that returned
`enum parse_fragment_result` only ever returned PF_OK or PF_ERROR.
Changing these functions to return bool simplifies things a lot. This
also helped in identifying some places where I didn't handle all
parse_fragment_result values properly.
- More cleaning up
- Allow linebreaks inside parenthesis
During development on the parser I got a "malloc(): corrupted top size"
error in the tokenizer when parsing `a=a`. I wrote this test to see if it
was really a problem with the tokenizer. It wasn't, lets keep the test
nontheless.
