Removed calls to deprecated items, other small updates.

Hey, I made some small changes to your library and updated some of the code to use non-deprecated items. Here's a run down of what I did:

* Stemmer uses bytes instead of ascii - gets rid of all the conversions (`to_char`, `as_ascii`, etc.), and makes the code more readable in my opinion.
* Updated tests to make them use assertions, added benchmarks.
* Attempted to make code more idiomatic rust (probably still needs work though).
* Replaced calls to deprecated items -- removed all warnings compiling with the most recent nightly build.
* Removed public visibility on Stemmer (made more sense to not expose it publicly to me)
* Attempted to add `inline` directive to some functions to improve performance.

Running the newly added bench marks, there seems to be a slight improvement in performance:

Old code: 
```
running 3 tests
test test_stem::lexicon ... ignored
test test_stem::lexicon_bench ... bench:   4680445 ns/iter (+/- 88446)
test test_stem::single_bench  ... bench:       180 ns/iter (+/- 7)
```

My pull:
```
running 3 tests
test test_stem::lexicon ... ignored
test test_stem::lexicon_bench ... bench:   4170162 ns/iter (+/- 82477)
test test_stem::single_bench  ... bench:       132 ns/iter (+/- 7)
```

could you please give reasoning why you'd like to hide this?