add missing `From` conversions to `SmallVec`

[alejandro-vaz]

those which have already into_* methods, but for some reason we don't have them as convert implementations

e.g.:

1. to Vec<T, A>
2. to Box<[T], A>

etc etc etc

that have methods but not the From trait conversion

[shuaixr]

Hi, can i work on this? but from what I can tell, it seems like only Vec and Box are suitable for From right now... Is that correct?

[alejandro-vaz]

yeah sure, I have assigned you

it may be that only those two are suitable for From, I haven't manually checked

make sure that you use the Vec and Box impls from the allocator API, that's what #632 tries to fix

we may also consider deprecating the old ones into_* since it's more idiomatic to simply use the trait