Use Hcx/hcx consistently for StableHashingContext.

The `HashStable` and `ToStableHashKey` traits both have a type parameter
that is sometimes called `CTX` and sometimes called `HCX`. (In practice
this type parameter is always instantiated as `StableHashingContext`.)
Similarly, variables with these types are sometimes called `ctx` and
sometimes called `hcx`. This inconsistency has bugged me for some time.

The `HCX`/`hcx` form is more informative (the `H`/`h` indicates what
type of context it is) and it matches other cases like `tcx`, `dcx`,
`icx`.

Also, RFC 430 says that type parameters should have names that are
"concise UpperCamelCase, usually single uppercase letter: T". In this
case `H` feels insufficient, and `Hcx` feels better.

Therefore, this commit changes the code to use `Hcx`/`hcx` everywhere.

Author: nnethercote

src/driver/aot.rs

@@ -45,8 +45,8 @@ enum OngoingModuleCodegen {
     Async(JoinHandle<Result<ModuleCodegenResult, String>>),
 }
 
-impl<HCX> HashStable<HCX> for OngoingModuleCodegen {
-    fn hash_stable(&self, _: &mut HCX, _: &mut StableHasher) {
+impl<Hcx> HashStable<Hcx> for OngoingModuleCodegen {
+    fn hash_stable(&self, _: &mut Hcx, _: &mut StableHasher) {
         // do nothing
     }
 }