Add into_inner method

[notgull]

This method adds the into_inner method to both ArrayVec and TinyVec and resolves #122. It also adds documentation and doc-tests for both of these functions.

Implementation questions:

• If the length of an ArrayVec is less than its capacity, should it return an error value or should it return the array with its default values? I have taken the latter option for the time being.

[Shnatsel]

Why not return the inner Vec instead of erroring out in TinyVec?

[notgull]

I was mostly just copying smallvec's way of doing it. However, it would certainly be possible to return an enum containing either an array or a vector. I'd like to hear Lokathor's thoughts on what the API should look like.

[Lokathor]

yeouch, lots of diff noise, guess we haven't formatted the repo in a little while.

For TinyVec, the inner fields are already public since it's just an enum, I don't think we need to have an into_inner method at all. People can already do a match and then handle the error non-array however they want.

[Lokathor]

great! though also the example doesn't need to be a whole page :3 just a few lines showing that you can get the array out should be enough. I can fix that before publishing if you don't want to.

not sure why CI stopped, i'll restart it