port `smallvec!` changes from #424 to v1

[alejandro-vaz]

see #424

[alejandro-vaz]

@bolshoytoster would you like to implement this

[bolshoytoster]

I'll have to benchmark it to see if it's actually worth it first.

[bolshoytoster]

There's no From<[T; M]> in v1, and I think it would require const generics to implement so I'm not sure if this can be done.

[alejandro-vaz]

@bolshoytoster then let's just keep it v2 code