The specific problem here is that our literal search doesn't know about
anchors, so it will try to search all of the detected literals in a regex.
In a regex like `a|^b`, the literal `b` should only be searched for at the
beginning of the haystack and in no other place.

The right way to fix this is probably to make the literal detector smarter,
but the literal detector is already too complex. Instead, this commit
detects whether a regex is partially anchored (that is, when the regex has
at least one matchable sub-expression that is anchored), and if so,
disables the literal engine.

Note that this doesn't disable all literal optimizations, just the
optimization that opts out of regex engines entirely. Both the DFA and the
NFA will still use literal prefixes to search. Namely, if it searches and
finds a literal that needs to be anchored but isn't in the haystack, then
the regex engine rules it out as a false positive.

Fixes rust-lang#268.

If the caller asks for captures, and the DFA runs, and there's a match,
and there are actually captures in the regex, then the haystack sent to
the NFA is shortened to correspond to only the match plus some room at the
end for matching zero-width assertions. This "room at the end" needs to be
big enough to at least fit an UTF-8 encoded Unicode codepoint.

Fixes rust-lang#264.

Docopt uses lazy_static! 2.x, but lazy_static required a new minimum
Rust version in 2.1.