Add Rust target_triple utilities, convert repositories.bzl to use, add platform(..) (#105)

* Add Platforms, Add rust triple handling tools

* Revert weird change

* fix bad import path, fix bad for loop

* Make template functions public

* Address comments

rust/BUILD

diff --git a/rust/BUILD b/rust/BUILD
index e9114f9..e67ccbf 100644
--- a/rust/BUILD
+++ b/rust/BUILD
@@ -1,7 +1,10 @@
 package(default_visibility = ["//visibility:public"])
 
 exports_files([
+    "known_shas.bzl",
+    "repositories.bzl",
     "rust.bzl",
     "toolchain.bzl",
-    "repositories.bzl",
+    "triple_mappings.bzl",
+    "utils.bzl",
 ])