feat(library/data/buffer/parser): add parser.{any_char,digit,nat} (#331)
feat(library/data/buffer/parser): add parser.{any_char,digit,nat} (#331)

Add three basic parsers:

- `any_char` consumes and returns any character
- `digit` parses a digit
- `nat` (naively) parses a natural number



leanprover-community/lean,digama0/lean,leanprover-community/lean,leanprover-community/lean,dselsam/lean,fpvandoorn/lean,fpvandoorn/lean,dselsam/lean,digama0/lean,leanprover-community/lean,fpvandoorn/lean,leanprover-community/lean,digama0/lean,digama0/lean,dselsam/lean,fpvandoorn/lean,dselsam/lean,dselsam/lean,fpvandoorn/lean,digama0/lean,dselsam/lean,digama0/lean,leanprover-community/lean,fpvandoorn/lean