Fix entailment notations `(⊢@{PROP})` and `(⊣⊢@{PROP} )` etc.

Fix #302, including their ASCII variants.
- Don't use quotes `'` that are not surrounded by spaces.
- However, notation `'(⊢@{' PROP } )` prevents parsing `(⊢@{PROP} Q)` using the
`⊢@{PROP} Q` notation.

To fix that, we force left-factorization: we add a notation for `'(⊢@{' PROP }
Q )`, defined to coincide with '⊢@{' PROP } Q but which can be left-factored
with `( '⊢@{' PROP } )`.

- Add left and right operator sections for (bi)entailment
- Add tests.

Also do all of the above also for ASCII notations, except for operator sections,
which seem to require more discussion.