This commits adds the support for dictionaries
(https://llvm.org/docs/LibFuzzer.html#dictionaries), to help fuzzers increase
their coverage faster.

It seems that there is a bug in the _copy function, because the word is
correctly inserted, but it seems that the padding after it is wrong,
and I couldn't understand why. Although to be honest,
I didn't spent much time on it, since I'd like to have feedback
on this PR before investing more debug time.

The implementation is pretty crude, it silently ignore
invalid lines in the dictionary file, and is likely using
words in the corpus a bit too often.