We'll soon need it elsewhere.
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.
Seems that we can parse most things now :). Assignments don't work yet, thoug. Also we're currently leaking memory pretty badly.
