WIP More syntactic improvements.

Relevant:

 - Make infix default.

 - Add semicolon pseudo-operator.

 - Change local syntax from "->{ x y ... }" to "-> x y ...;".

 - Add section syntax:

       (_ - 5)    5 (-)
       (5 - _)    5 swap (-)
       (_ - _)    (-)

 - Refactor the prelude accordingly.

Earmarked:

 - Remove unit type and syntax.

 - Remove unused 'Prelude_Unit' and 'Prelude_Effect' modules.

Yet to do:

 - Enforce invariants.

 - Improve error messages.

 - Allow "_" placeholder as local name to drop a value.