Add a "fast_wrap" function that reuses the input string

[mgeisler]

The current wrap method allocates completely new strings for each line. This is necessary is one wants to support automatic hyphenation since the output can be different from the input in that case.

However, for the simple case with no hyphenation, it should be possible to return Vec<&str> where each &str is a slice of the original input string. This would mean fewer allocations and no copying of data, which ought to be faster.

[mgeisler]

Fixed by #69.