Composite styles

[gpeairs]

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.