A couple of notes:
 - Populated instances of Forward are represented only as "Forward(...)"
   to prevent infinite recursion.
 - Helper classes like _PendingSkip and _ErrorStop are not yet handled.

Introduces a _make_repr() method with parameters to account for the
different contexts in which an element may need to be represented (top
level or as a term in an expression, as a potential string literal or
not).  In order to make the code a little easier to read, the
__format__() method is implemented as well, with conversion specifiers
used to indicate the context.

This function determines how to represent a set of characters using
srange(), then returns either the original set or the srange expression,
whichever is shorter.  To be used in repr() implementations in elements,
such as Word, which are based on sets of characters.