libbpf-rs: program: Show AsFd in documentation

Previously, AsFd trait was implemented on ProgramImpl. Since ProgramImpl
is not exported, the trait implementation did not show up on docs.rs.
You could still use it - but it wasn't obvious that it was available.

libbpf-rs/src/program.rs

@@ -564,6 +564,13 @@ pub struct ProgramImpl<'obj, T = ()> {
     _phantom: PhantomData<&'obj T>,
 }
 
+impl<T> ProgramImpl<'_, T> {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        let fd = unsafe { libbpf_sys::bpf_program__fd(self.ptr.as_ptr()) };
+        unsafe { BorrowedFd::borrow_raw(fd) }
+    }
+}
+
 impl<'obj> Program<'obj> {
     /// Create a [`Program`] from a [`libbpf_sys::bpf_program`]
     pub fn new(prog: &'obj libbpf_sys::bpf_program) -> Self {
@@ -706,6 +713,12 @@ impl<'obj> Program<'obj> {
     }
 }
 
+impl AsFd for Program<'_> {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        ProgramImpl::as_fd(self)
+    }
+}
+
 impl<'obj> ProgramMut<'obj> {
     /// Create a [`Program`] from a [`libbpf_sys::bpf_program`]
     pub fn new_mut(prog: &'obj mut libbpf_sys::bpf_program) -> Self {
@@ -1153,6 +1166,12 @@ impl<'obj> ProgramMut<'obj> {
     }
 }
 
+impl AsFd for ProgramMut<'_> {
+    fn as_fd(&self) -> BorrowedFd<'_> {
+        ProgramImpl::as_fd(self)
+    }
+}
+
 impl<'obj> Deref for ProgramMut<'obj> {
     type Target = Program<'obj>;
 
@@ -1163,13 +1182,6 @@ impl<'obj> Deref for ProgramMut<'obj> {
     }
 }
 
-impl<T> AsFd for ProgramImpl<'_, T> {
-    fn as_fd(&self) -> BorrowedFd<'_> {
-        let fd = unsafe { libbpf_sys::bpf_program__fd(self.ptr.as_ptr()) };
-        unsafe { BorrowedFd::borrow_raw(fd) }
-    }
-}
-
 impl<T> AsRawLibbpf for ProgramImpl<'_, T> {
     type LibbpfType = libbpf_sys::bpf_program;