memquota features, preparation

• Change the name of the feature that enables the HasMemoryCost trait, in crates where that's conditionally-compiled, to memquota-memcost. That leaves memquota for "we actually enabale the quota system". Using the same name for these two things was confusing.

• Have everyone that depends on tor-memquota for its traits and types use default-features = false, so we can make tor-memquota/memquota enabled by default.

This is the textually large but quite boring prefix of a branch where I actually document things and sort out the features. I'd appreciate a quick turnaround as this branch is likely to rot quickly.