Remove http header re-exports from crate root

[yoshuawuyts]

We currently export the following headers from the crate root:

A majority of these are re-exports from our sub-modules. As our index is growing because of the addition of a variety of typed headers I would like to trim down some of these re-exports and leave them accessible only as part of the sub-modules. The types to keep would be only the types required to construct a request / response, or types that don't currently already have a dedicated submodule:

keep

• Body core type
• Error core type
• Extensions no dedicated submodule
• Request core type
• Response core type
• Url core type (for now; revisit after the TryFrom patch on url is published)

move to submodule

• Cookie has submodule, not critical
• Headers rarely referenced directly, and when used it's in conjunction with other header types
• Mime (rename to MediaType (see #251 )
• Trailers has a submodule and uncommonly used

[brightly-salty]

I'd be willing to attempt to implement this and submit a PR. Is this what is needed?

• remove pub use crate::cookies::Cookie from src/lib.rs
• remove pub use mime::Mime from src/lib.rs
• remove pub use trailers::Trailers; from src/lib.rs
• remove pub use headers::Headers from src/lib.rs

[yoshuawuyts]

@brightly-salty yes indeed!

[joshtriplett]

Fixed by #305.