Add a RawSmallVec type

[timothee-haudebourg]

I see a lot of suggestions such as #220 and #226 to add new data structures based on small vec. I believe your position is to let other crates provide these data structures, which is great. However this can be a bit tedious with the current api which lacks a way to deal with "small" memory chunks independently of the actual data stored on it.

In the standard library, this is done using the RawVec type. Would it be possible to provide a similar RawSmallVec type that would allow other crates to build on that to create other data structures such as SmallVecDeque? This could be implemented in the 2.0 version perhaps.

[mbrubeck]

Good idea. I think this fits in well with the 2.0 plan in #240.