Make VarBitSet saturated + FromIterator efficient

[sameer]

I noticed that VarBitSet::saturated takes a significant amount of time when generating data with VecStrategy. There are two perf issues:

1. not making use of the size hint in FromIterator leads to repeated grow calls
2. saturated should use from_elem since we're setting all bits

[sameer]

I'm using Instruments on macOS to look at the CPU profile of a proptest and it's now optimized to memset 🚀

Before:

After:

[sameer]

@matthew-russo are you able to approve the workflows / merge this?

[matthew-russo]

I didn't realize the workflows weren't running. just kicked that off. we normally try to get two maintainers eyes on things before merging -- i pinged the others yesterday to give a look.