Make mopa optional

[willglynn]

std::any::Any ought to be suitable for most users. If someone knows they want Component to require mopa::Any, they should opt-in via a feature flag.

mopa::Any describes itself as:

This is a simple wrapper around std::any::Any which exists for technical reasons. Every type that implements std::any::Any implements this Any.

std::any::Any gained downcast_ref() and type_id() in the years since mopa was released. mopa::Any also doesn't perform any function inside specs, so unless a user asks for specs::Component to have that dependency, I think it's reasonable to use std::any::Any instead.

Checklist

• n/a I've added tests for all code changes and additions (where applicable)
• n/a I've added a demonstration of the new feature to one or more examples
• n/a I've updated the book to reflect my changes
• n/a Usage of new public items is shown in the API docs

Nothing in the specs repo except the Component definition and an associated generic type in a test refer to Any, and even then only in type bounds. No tests or examples are affected.

API changes

This change breaks any code which specifically needs trait Component: mopa::Any by requiring them to use the new mopa feature:

specs = { version = "*", features = ["mopa" ] }

I added a changelog entry to that effect.

[willglynn]

Actually, nevermind. mopa requires both pub trait Component: mopa::Any and mopafy!(Component). specs doesn't call that macro for Component right now – its mopa support is broken – and it didn't call it for Component even back when mopa was first added. I'll open a new PR to remove mopa completely.