Unsoundness in Entry methods

[Anders429]

Entry::add() and Entry::remove() both currently assume the components specified exist within the Registry, when that can't actually be assumed. Rather than doing an unwrap_unchecked on the component_index at the beginning of both methods, a regular unwrap should be done to trigger a panic if the component is not part of the Registry.

In the future, hopefully we can move this check to compile-time. Ideally, we'd be able to check if a component is a part of a Registry using a const fn, but that is currently only possible in nightly Rust. Perhaps it may be worthwhile to introduce a nightly feature gate to allow const checks.