Hide unsafe words base

[oscbyspro]

The unsafe words API exposes its underlying Unsafe[Mutable]Pointer<UInt> and the only reason it does this, or at the least the main reason, is so that I can call:

words.base.initialize(repeating: 0, count: words.count)

This is a bit lazy, however, and not in the spirit of keeping "[hashtag] good form [trademark]" (#79) because the initialize/update concept are not otherwise exposed (since UInt is trivial).

[oscbyspro]

This does the same thing, so it's not like the convenience is worth much.

for index in words.indices {
    words[index] = 0
}

[oscbyspro]

Unrelated, but I’m not convinced _read and _modify are needed on a UInt pointer.