Use MaybeUninit while manipulating uninitialized data

[vorner]

Because there's no way to manipulate unitialized data to some data types
just through references.

Adding Arena::reserve_extend to allow a somewhat safer pattern how to
initialize the memory without risk of causing UB by panicking before
full initialization, causing the arena to call destructors on bad data.

Closes #38.

If the documentation need improving or some more tests are need, just point them out please. I'm sometimes blind to these things.

Also, I hope I've been careful enough. I kind of assume that Vec already assumes the buffer might contain some uninitialized values so it's fine to cast them to MaybeUninit and hand out pointers to such things.

[fitzgen]

Great docs! Thanks!

[fitzgen]

Published 2.0.0-rc1 -- if we don't have any issues discovered or raised in the next week or so, then I'll publish 2.0.0 proper.

[vorner]

Thanks. I've noticed now that the docs are rendered on docs.rs that the links to methods don't work. I'll send a fix for that (I think I can make that one before the release).