Make FixedLengthSource public API

[Gaming32]

I know it's trivially reimplementable, but it still feels frustrating to reinvent the wheel when there's a perfectly good wheel that's internal. (I actually just copied the class into my own project, so there are two now, one internal to okio, and one in my project.)

[JakeWharton]

I'm pretty sure we were supposed to expose this like 5 years ago. I'm not sure what happened. Probably my fault.

We agreed to expose it but I'm not sure if we settled on the API. I think it's probably something like Source.limit(long): Source which gives you a wrapped source that doesn't let you read past n bytes.

[swankjesse]

Okay this is pretty awesome. The G in your profile pic is from the Chlorinap font that I made in 1998. To celebrate this fun coincidence I shall rush this new API.