Backreferences to non-participating groups are handled incorrectly

[jaynetics]

Issue extracted from #21.

E.g. ruby does this:

/\1()/.match?('foo') # => false

but the resulting js regex, with the same source, produces a zero-length match on any string.

[slevithan]

A few other examples of regexes that include nonparticipating capturing groups:

• (\1) - should fail to match
• (a)|b\1 - should never match the second alternative
• (?<n>a)|(?<n>)\k<n> - the backreference should not allow matching an empty string as one of the multiplexed options (see No backref multiplexing #28)

Might include a nonparticipating capturing group:

• (a)?b\1
• ((a)|b)+\2

Never includes a nonparticipating capturing group:

• (a?)b\1