Separate lifetime Parser and WootheeResult

[hhatto]

for #5

@pjenvey
What about this change?

FYI: Performance will decrease slightly.

$ cargo benchcmp bench.old bench.new
 name              bench.old ns/iter  bench.new ns/iter  diff ns/iter  diff %  speedup
 bench_stabilizer  33                 34                            1   3.03%   x 0.97
 smoke_woothee     10,698             11,753                    1,055   9.86%   x 0.91

[pjenvey]

@hhatto thanks for addressing this. However I'm not sure you need to convert everything to heap allocated strings to solve this. Granted, I haven't looked at every bit of the parser code, but AFAICT what were the original &str's were all 'static, or possibly the lifetime of the original agent &str.

I think the various parser methods may just need more careful handling of lifetimes, in particular, passing around the lifetime of the original agent: &str and tying it to WootheeResult's lifetime (whereas the 'static &strs can always be coereced to this shorter lifetime)