`const` checks

There are some checks that can be evaluated at compile time, rather than runtime. These are:

- [x] entity insertion: ensure components are all members of the registry.
- [x] entry add: ensure component is member of registry.
- [x] entry remove: ensure component is member of registry.
- [x] query: ensure components are not borrowed mutably while being borrowed elsewhere within a view.
- [ ] registry: ensure there are no duplicate components.

These are all programmer errors, and should be made known as soon as possible.

These checks can be moved to compile-time using `const fn`s. For example, a check could be run as follows: `const _: () = some_check()`. However, all of these cases will currently require unstable nightly features. Therefore, I think it best to introduce a `unstable_const_checks` feature that requires `nightly`, allowing users to opt-in to these checks instead of the run-time checks.

Related issues: #40 and #27.