* Consolidate and freeze symbols
Instead of having symbol values defined in three potentially conflicting
places (`spacy.attrs`, `spacy.parts_of_speech`, `spacy.symbols`), define
all symbols in `spacy.symbols` and reference those values in
`spacy.attrs` and `spacy.parts_of_speech`.
Remove deprecated and placeholder symbols from `spacy.attrs.IDS`.
Make `spacy.attrs.NAMES` and `spacy.symbols.NAMES` reverse dicts rather
than lists in order to support future use of hash values in `attr_id_t`.
Minor changes:
* Use `uint64_t` for attrs in `Doc.to_array` to support future use of
hash values
* Remove unneeded attrs filter for error message in `Doc.to_array`
* Remove unused attr `SENT_END`
* Handle dynamic size of attr_id_t in Doc.to_array
* Undo added warnings
* Refactor to make Doc.to_array more similar to Doc.from_array
* Improve refactoring
