libbpf-rs: Generate docs for impls on type alias

rustdoc apparently cannot generate docs for methods implemented on a
type alias target unless said type alias is exported from the crate. I
think it's b/c under the hood rustdoc is using some clever javascript
tricks to inline documentation. They do this to avoid HTML bloat for
crates with lots of type alises [0].

The upshot of this is now trait impls like `AsFd` are visible in web
docs. This is really important, as otherwise it's quite difficult for
users to discover these APIs.

The downside of this approach is we publically expose implementation
structs in docs. It's not a huge deal, as with skeleton it should be
fairly obvious what types they should actually interact with. I kinda
doubt anyone reads the docs front to back.

[0]: rust-lang/rust#116471

libbpf-rs/src/lib.rs

@@ -112,11 +112,13 @@ pub use crate::map::Map;
 pub use crate::map::MapCore;
 pub use crate::map::MapFlags;
 pub use crate::map::MapHandle;
+pub use crate::map::MapImpl;
 pub use crate::map::MapInfo;
 pub use crate::map::MapKeyIter;
 pub use crate::map::MapMut;
 pub use crate::map::MapType;
 pub use crate::map::OpenMap;
+pub use crate::map::OpenMapImpl;
 pub use crate::map::OpenMapMut;
 pub use crate::object::AsRawLibbpf;
 pub use crate::object::MapIter;
@@ -132,10 +134,12 @@ pub use crate::print::PrintCallback;
 pub use crate::print::PrintLevel;
 pub use crate::program::Input as ProgramInput;
 pub use crate::program::OpenProgram;
+pub use crate::program::OpenProgramImpl;
 pub use crate::program::OpenProgramMut;
 pub use crate::program::Output as ProgramOutput;
 pub use crate::program::Program;
 pub use crate::program::ProgramAttachType;
+pub use crate::program::ProgramImpl;
 pub use crate::program::ProgramMut;
 pub use crate::program::ProgramType;
 pub use crate::program::TracepointOpts;