Ghostwriter: create @composite strategy from type

[cleder]

Writing composite strategies is a bit of a chore.
The hypothesis.strategies.from_type is great, but to customize it is a bit complex.

It would be great if one could generate composite strategies sourcecode from a type.
(this also may be a workaround for #2876 )

[Zac-HD]

Can you show some examples of what you think Ghostwriter could output, and how you'd build on that?

I'd expect st.builds(T, ...) to work pretty well in most cases; you only need @st.composite if there are dependencies between arguments.