Simplify definition of left and right unitors (natural transformation).

Shorten names:
  - prebicategory_ob_1mor_2mor  -> prebicategory_ob_hom
  - identity_1mor -> identity1
  - identity_2mor -> identity2
  - composition_1mor -> composition1
  - compose_2mor_horizontal -> compose2h
  - compose_2mor_iso_horizontal -> compose2h_iso

Remove superfluous definition
  identity_2mor (just use identity)

Remove unused code
  idto1mor

Add explict type annotations in statemes.

Notations
   - new scope "bicategories" (thus remove Local Notations);
   - open cat scope (and use it extensively);
   - make notations '-2->' and ':v;' specific for bicategories
     (otherwise they would overlap with the already available notations for
     categories);
   - remove notation '-2->' (duplicate of '-->');
   - fix spaces in notations;

Formatting
  - extensive use of (unicode) notations;
  - indentation, trim spaces;
  - superfluous parenthesis;
  - etc, etc.
