implementation of fmt

I've got an initial implementation of fmt completed. The options are sort of a combination of GNU and BSD, with a couple extra minor ones that I added because it seemed sensible and relatively straightforward.

Note: for now, this version does not use Knuth-Plass, but everything else is in place using "greedy" breaking. I've got a working Knuth-Plass implementation in Haskell that I'm going to port over.

All options (should) work, and performance is nearly on par with GNU fmt.

(GNU fmt is the first run, this version is the second.)

    $ ls -lah playground/long
    -rw-r--r-- 1 kwantam kwantam 42M Jun 18 20:51 playground/long
    $ perf stat -r 20 fmt playground/long >/dev/null
    
     Performance counter stats for './fmt playground/long' (20 runs):
    
           1077.217547      task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.34% )
                    12      context-switches          #    0.011 K/sec                    ( +- 12.48% )
                     3      cpu-migrations            #    0.003 K/sec                    ( +- 27.58% )
                   184      page-faults               #    0.171 K/sec                    ( +-  0.04% )
         3,485,343,206      cycles                    #    3.236 GHz                      ( +-  0.06% )
           833,519,491      stalled-cycles-frontend   #   23.91% frontend cycles idle     ( +-  0.20% )
       <not supported>      stalled-cycles-backend   
         7,521,678,405      instructions              #    2.16  insns per cycle        
                                                      #    0.11  stalled cycles per insn  ( +-  0.00% )
         1,439,929,997      branches                  # 1336.712 M/sec                    ( +-  0.00% )
            40,051,616      branch-misses             #    2.78% of all branches          ( +-  0.01% )
    
           1.078562530 seconds time elapsed                                          ( +-  0.34% )

    $ perf stat -r 20 ./fmt playground/long >/dev/null

     Performance counter stats for './fmt playground/long' (20 runs):
    
           1267.654011      task-clock (msec)         #    0.999 CPUs utilized            ( +-  0.23% )
                    10      context-switches          #    0.008 K/sec                    ( +-  7.02% )
                     2      cpu-migrations            #    0.002 K/sec                    ( +- 43.83% )
               101,079      page-faults               #    0.080 M/sec                    ( +-  0.00% )
         4,125,868,082      cycles                    #    3.255 GHz                      ( +-  0.03% )
           896,450,011      stalled-cycles-frontend   #   21.73% frontend cycles idle     ( +-  0.05% )
       <not supported>      stalled-cycles-backend   
         9,669,115,061      instructions              #    2.34  insns per cycle        
                                                      #    0.09  stalled cycles per insn  ( +-  0.00% )
         2,302,891,543      branches                  # 1816.656 M/sec                    ( +-  0.00% )
            19,958,464      branch-misses             #    0.87% of all branches          ( +-  0.00% )
    
           1.268852889 seconds time elapsed                                          ( +-  0.23% )

These statements should probably be separated on new lines like:
```
Some(s) => {
    fmt_opts.prefix = s;
    fmt_opts.use_prefix = true;
    fmt_opts.prefix_len = fmt_opts.prefix.as_slice().char_len();
}
```