Make ComponentDescriptor const

## What problem does this solve or what need does it fill?

The `ComponentDescriptor` is always the same for each component type.

## What solution would you like?

Create an associated constant on the `Component` and `Resource` trait, which stores the existing information.

Constify all of the methods on `ComponentDescriptor`, then use those to initialize this information at compile-time.

We can also do this for `NonSend` resources by creating a simple trait for them.

## What alternative(s) have you considered?

Enable the feature flags and do it now!

## Additional context

This is blocked by several (!) unstable `rustc` features.

1. trait bounds other than `Sized` on const fn parameters are unstable
2. `std::any::type_name` is not yet stable as a const fn
3. `TypeId::of` is not yet stable as a const fn
4. function pointer casts are not allowed in constant functions

https://github.com/rust-lang/rust/issues/57563 is the relevant tracking issue