[rust] Add rust_bench_test and rust_doc_test rules and improve usability of rust_test rule.

* Add rust_bench_test rule to run benchmark tests
* Add rust_doc_test rule to run Rust documentation tests.
* Enable rust_test and rust_bench_test to depend directly on a rust_library target.
* Rename rust_docs rule to rust_doc for consistency.

RELNOTES: [rust] Add rust_bench_test and rust_doc_test rules and improve usability of rust_test tule.

--
MOS_MIGRATED_REVID=104648497
diff --git a/WORKSPACE b/WORKSPACE
index b3aebe8..f9d107a 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -47,6 +47,7 @@
     url = "https://static.rust-lang.org/dist/rust-1.3.0-x86_64-unknown-linux-gnu.tar.gz",
     sha256 = "fa755b6331ff7554e6e8545ee20af7897b0adc65f471dd24ae8a467a944755b4",
     build_file = "tools/build_rules/rust/rust.BUILD",
+    strip_prefix = "rust-1.3.0-x86_64-unknown-linux-gnu",
 )
 
 new_http_archive(
@@ -54,6 +55,7 @@
     url = "https://static.rust-lang.org/dist/rust-1.3.0-x86_64-apple-darwin.tar.gz",
     sha256 = "bfeac876e22cc5fe63a250644ce1a6f3892c13a5461131a881419bd06fcb2011",
     build_file = "tools/build_rules/rust/rust.BUILD",
+    strip_prefix = "rust-1.3.0-x86_64-apple-darwin",
 )
 
 # In order to run the Android integration tests, uncomment these rules, point