Bug fix: `n-1` was scanned as `n` and `-1`, which was rejected by
a style constraint.
I removed useless style constraints in the lexer.
I removed dead code in `pascaligo.ml`.
I added my first draft for the reference manual of PascaLIGO: DO
NOT EDIT. Edit the website instead.
About the bug fix: This was an awkward attempt at rejecting at
lexing time negative integer literals whose sign is separated
from the digits, like `- 1`. The fix was simple: remove the
`integer` regular expression and use `natural`.
