py/compile: Refactor handling of special super() call.

This patch refactors the handling of the special super() call within the
compiler.  It removes the need for a global (to the compiler) state variable
which keeps track of whether the subject of an expression is super.  The
handling of super() is now done entirely within one function, which makes
the compiler a bit cleaner and allows to easily add more optimisations to
super calls.

Changes to the code size are:

   bare-arm: +12
    minimal:  +0
   unix x64: +48
unix nanbox: -16
     stmhal:  +4
     cc3200:  +0
    esp8266: -56