Implement special form of & predicate

[tmc]

&{ expression }

          In  this  predicate  the  simple C expression (not statement) is
          evaluated immediately when the parser reaches the predicate.  If
          the  expression  yields non-zero (true) the 'match' succeeds and
          the parser continues with the next element in the  pattern.   If
          the  expression  yields  zero  (false) the 'match' fails and the
          parser backs up to look for an alternative parse of the input.

Via http://piumarta.com/software/peg/peg.1.html,

[Archs]

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