Composite styles

Copying Hugh's description from the pre-release tracker:

Presently the mechanism for applying a composite style is through repeated call to `styled`: `styled(styled(ent, sty_lower), sty_upper)`, which results in `StyledEntity` that has a `StyledEntity` for it's `ent` field.

There are a few difficulties with this:
1) It isn't possible to store such a combination upfront to apply to multiple entities, this clashes with the design of `StyleDict`.
2) When applying compositions of styles such as `Optional(Rounded())` and `Optional(MeshSized())`, the resolution of `to_primitives` can result in ambiguous combinations. The options in this scenario are additive, if the `Rounded` is active, the primitive changes, and if the `MeshSized` is present then `meshgrading` and `meshsize` change.

It would be ideal to allow something like `sty = Rounded() ∘ MeshSized()` to construct such styles.

Start by tracing the existing styled calls and the StyledEntity and StyleDict concepts described in the issue, then inspect how to_primitives resolves Optional(Rounded()) and Optional(MeshSized()). Done should provide a reusable composition of styles that can be applied to multiple entities without ambiguous primitive resolution.

Written by the indexing model from the issue text.