refactor: simpler raw

[alejandro-vaz]

this PR moves the allocator from RawSmallVec to SmallVec, replacing RawSmallVec for the old RawSmallVecInner, and adding the allocator directly to IntoIter

it allows us to have a simpler internals API with a whole struct less, everything else stays the same

a safety guarantee to RawSmallVec::try_grow_raw was added, that the allocator must be the same the data is allocated in

closes #635

it also adds a Default for SmallVec<Type, N> implementation to make clippy stfu

key idea: RawSmallVec owns its data but not where it is (inline vs heap) nor where the heap is allocated (allocator)

[bolshoytoster]

Do we know of anyone actually using/planning to use the internals feature? This might be annoying to those people, if they exist.

Other than that, I do prefer self.raw.heap to self.raw.inner.heap.

[alejandro-vaz]

there are people interested in using the internals feature, you can look in issues at around #300 to #400, you'll probably find some

they were things like SmallVecDeque and LiteMap and some other things like that

it doesn't annoy anyone, brother the first version with this kind of internals was shipped yesterday hahahha

[alejandro-vaz]

indeed internals dates back to #348