`From<Vec<T>>` impl for `TinyVec`

[lopopolo]

Hi :wave:

I maintain a crate that abstracts over growable vectors such that they would be appropriate for implementing a Ruby Array. This crate had backends for Vec and SmallVec. Yesterday I added a backend for TinyVec.

PR is here: https://github.com/artichoke/artichoke/pull/1094.

I based the TinyVec backend on the existing backend for SmallVec. Apart from adding a boatload of T: Default bounds, the change was actually really small -- https://github.com/artichoke/artichoke/commit/1935c0543a9c26176494377da8c351a66a181eca.

I appreciated the increased API compatibility with Vec compared to SmallVec, in particular a splice method, which let me simplify several methods.

There were two things I was missing:

• There is no equivalent of vec! or SmallVec::from_elem with a non-constant length. I had to use iter::repeat like this:

  Self(iter::repeat(default).take(len).collect())

• There is no From<Vec<T>> impl, which forces one to go through vec::IntoIter even if the TinyVec would be spilled with this Vec.

[Lokathor]

Those both sound useful.

The impl sounds easy, but the macro one might be harder to slot in because we've already got quite a few macro arms, but you could probably fit it in.