Add std feature

The use of `std::collections::VecDeque` is the only thing in the way of `no_std` support. A default `std` feature should be added, without which the underlying queue will be some `no_std` queue.

Inspect the current use of std::collections::VecDeque and the project's feature configuration first. Confirm how the underlying queue is selected, then run the existing test suite and verify that the default build uses std while a no_std build uses the alternative queue.

Written by the indexing model from the issue text.