It's really easy to accidentally pass an uninitialized string as dst into
the copy function, which will result in an free() call to an arbitrary
pointer. Maybe it's a better idea to not deinit the dst string before
copying? The documentation at least makes it more clear and the new
apfl_string_blank() function makes it easy to create an empty string.
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.
