Add SmallVec::extract_if to match Vec::extract_if

[GnomedDev]

Currently, there exists the unstable Vec method extract_if which can be super useful. If this can be ported to SmallVec, that would be lovely and help with migration.

[mbrubeck]

Note: This is included in smallvec 2.0, which is currently available as a pre-release alpha version.

smallvec 1 has a similar method drain_filter which can be enabled using an optional Cargo feature:

https://docs.rs/smallvec/1/smallvec/index.html#drain_filter

(You can ignore the warning that the feature is unstable, since smallvec 1 is now in maintenance mode and will be receiving bug fixes only.)