Add fast paths to fill and wrap

[mgeisler]

In case the input is shorter than the wrapping width, we can return it immediately. This avoids findings words, breaking them, and finally reassembling them.

The speedup from the is on the order of 10-25 times, depending on the wrapping width. I tested with a wrapping width of 60 columns and there the time for fill("") is now around 15 nanoseconds. It was around 150 nanoseconds before:

String lengths/fill_first_fit/0000
                        time:   [15.843 ns 15.880 ns 15.936 ns]
                        change: [-90.123% -90.066% -90.003%] (p = 0.00 < 0.05)
                        Performance has improved.

A similar 10x improvement is seen for fill("abcde"):

String lengths/fill_first_fit/0005
                        time:   [28.256 ns 28.311 ns 28.385 ns]
                        change: [-90.482% -90.428% -90.373%] (p = 0.00 < 0.05)
                        Performance has improved.

As strings get longer and closer to the wrapping width, the improvements get larger. Before it took just shy of a microsecond to wrap a 50 character string on my machine:

String lengths/fill_first_fit/0050
                        time:   [943.91 ns 944.96 ns 946.17 ns]

Now it only takes 34 nanoseconds, a 27x improvement:

String lengths/fill_first_fit/0050
                        time:   [34.438 ns 34.506 ns 34.586 ns]
                        change: [-96.362% -96.336% -96.306%] (p = 0.00 < 0.05)
                        Performance has improved.

The time needed to wrap the input is nearly flat until you hit the wrapping width, as which time it grows linearly with the input length: