Improve why3doc output.
Improve why3doc output.

- Properly indent comments placed after a symbol (à la ocamldoc).
- Handle tabulations as spaces.
- Remove spurious blank lines with (***.
- Avoid the costlier fprintf whenever possible.

jpdeplaix/why3,jpdeplaix/why3,douglas-larocca/why3,ssaavedra/why3,douglas-larocca/why3,jpdeplaix/why3,ssaavedra/why3,ssaavedra/why3,douglas-larocca/why3