vec: add extract_if()

[max-heller]

Adapts alloc::vec::Vec::extract_if() to implement heapless::Vec::extract_if()

I noticed that vec::Drain stores LenT instead of usize for its internal indices. Should we do the same here to save a few bytes? I wasn't sure if it's worth it relative to the (also small) cost of casting back and forth between LenT and usize.

[hiSandog]

Worth using LenT here for consistency — the existing vec::Drain in the crate already stores its indices as LenT, and since heapless::Vec's own length is bounded by LenT anyway, the retained/removed range indices can never exceed LenT::MAX, so the narrowing is always safe. The LenT as usize casts on indexing aren't a perf concern: extract_if's indices drive slice bounds and drop_in_place bookkeeping, not a hot arithmetic loop, and on most targets the cast compiles to nothing. Main thing to watch is keeping the kept-range start/end and the read/write cursors all on LenT (not mixed), so you don't end up casting in two directions. Keeping the two drain-like types (Drain and ExtractIf) on the same index representation is a nice consistency win too.