make the module containing mock fields pub so it shows up in rustdocs

Summary:
the rustdocs for mocks can be a bit confusing because there's no documentation for any of the individual mock fields. this is because the module in which the mock field objects are defined is private.

to expose them in rustdoc, just `pub` that module. each of the mock field objects have private fields, so downstream crates will still be unable to instantiate these mock fields on their own (and this makes `unimplemented` `pub(crate)` to ensure that as well).

the link to some mock fields is still broken, e.g. `FacebookService`, but that's because of rust-lang/docs.rs#1588 (comment).

Reviewed By: thepacketgeek

Differential Revision: D45406779

fbshipit-source-id: 91f14ef2e022857969540c82d2482e81bda1421c

Author: emersonford

third-party/thrift/src/thrift/compiler/generate/templates/rust/lib/mod.mock.mustache

@@ -111,7 +111,7 @@
 
     {{/service:extendedServices}}{{!
     }}{{/program:services}}
-    mod r#impl {{>lib/block}}{{!
+    pub mod r#impl {{>lib/block}}{{!
     }}{{#program:services}}
         pub mod {{service:snake}} {{>lib/block}}{{!
 
@@ -134,7 +134,7 @@
 
             #[allow(clippy::redundant_closure)]
             impl<'mock> {{function:rust_name}}<'mock> {
-                pub fn unimplemented() -> Self {
+                pub(crate) fn unimplemented() -> Self {
                     Self {
                         closure: ::std::sync::Mutex::new(::std::boxed::Box::new(|{{!
                             }}{{#function:args}}{{!