Splits up into:
- escaping: resolving of escape sequences
- parser: the parsing code
- tests: all integrated parsing tests
Also moves Ident from the root syntax module into the tree module.
This basically reverts the earlier change from parbreaks to par nodes because:
- It is simpler and less nested
- It works way better with functions that layout their body inline like `font`, which where buggy before, previously
The original reasons for changing to par nodes were:
- the envisioned design of the layouter at that time (based on dynamic nodes etc.), which is not relevant anymore
- possibly existing benefits with regards to incremental compilation, which are unsure and outweighed by the immediate benefits of the parbreak-representation
Doesn't layout contents into a box anymore, instead layouting inline in the parent context. Also makes axis inferring for center alignents smarter (just because I had fun doing it). It's unsure whether we want to keep it because it might be confusing.
- Refactors the tokenizer to be lazy: It does not emit pre-parsed function tokens, but instead allows it's mode to be changed. The modes are tracked on a stack to allow nested compute/typesetting (pop/push).
- Introduces delimited groups into the parser, which make it easy to parse delimited expressions without handling the delimiters in the parsing code for the group's content. A group is started with `start_group`. When reaching the group's end (matching delimiter) the eat and peek methods will simply return `None` instead of the delimiter, stopping the content parser and bubbling up the call stack until `end_group` is called to clear up the situation.
