Implement `System` for `Box<dyn System>`

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

Boxed systems cannot be used with `IntoSystem`. This means that they can't use `IntoSystem::pipe` or any of the other related functions. This is especially useful for downstream crates relying on user-provided type erased systems.

## What solution would you like?

Implement `System` for `Box` and both `System` and `ReadOnlySystem` for `Box`

## What alternative(s) have you considered?

N/A

Start by locating the System and ReadOnlySystem trait definitions and the IntoSystem APIs mentioned in the issue. Check the existing trait implementations around boxed or type-erased systems, then verify that the requested boxed types work with IntoSystem::pipe and related functions.

Written by the indexing model from the issue text.