Generator: bound value preservation analysis

Generator: bound value preservation analysis

Value-preservation analysis recursively examines the upcoming expression before
the generator decides whether mutable-slot operands need temporary copies.
Deeply nested binary expressions and arrays could therefore exhaust the native
stack.

Move the generator-specific predicate out of the parser API and limit its
recursive walk to 128 levels.  At the limit, conservatively require
preservation: an extra move is safe, whereas missing a deeper side effect can
change evaluation semantics.