For cleanliness the prelude module only needs to reexport preludes from
direct, "top-most" crates, which themselves take care of reexporting
preludes from its dependencies again. This shaves off some code while
maintaining the same set of exports.
In gir it was brought up [1] that some traits (in particular
`*ExtManual`) are exported from the crate root in addition to the
prelude, cluttering the environment unnecessarily. This commit removes
all these reexports, leaving those in prelude (that were already there)
only.
After this commit everything matching `Ext(Manual)?\b` in `lib.rs` sits
within `pub mod prelude {};`.
[1]: https://github.com/gtk-rs/gir/pull/1111
The next version of gir is going to generate doc(cfg()) attributes on
many symbols to show feature-dependence hints. While autogenerated sys
crates get this attribute in their own (generated) lib.rs file the safe
wrapper crates do not have such an autogenerated lib.rs file.
