Change the name of wit-bindgen's host implementation traits. (#5890)

* Change the name of wit-bindgen's host implementation traits.

Instead of naming the host implementation trait something like
`wasi_filesystem::WasiFilesystem`, name it `wasi_filesystem::Host`, and
avoid using the identifier `Host` in other places.

This fixes a collision when generating bindings for the current
wasi-clock API, which contains an interface `wall-clock` which contains
a type `wall-clock`, which created a naming collision on the name
`WallClock`.

* Update tests to use the new trait name.

* Fix one more.

* Add the new test interface to the simple-wasi world.

crates/wit-bindgen/src/rust.rs

@@ -368,6 +368,7 @@ pub trait RustGenerator<'a> {
     }
 }
 
+/// Translate `name` to a Rust `snake_case` identifier.
 pub fn to_rust_ident(name: &str) -> String {
     match name {
         // Escape Rust keywords.
@@ -425,3 +426,13 @@ pub fn to_rust_ident(name: &str) -> String {
         s => s.to_snake_case(),
     }
 }
+
+/// Translate `name` to a Rust `UpperCamelCase` identifier.
+pub fn to_rust_upper_camel_case(name: &str) -> String {
+    match name {
+        // We use `Host` as the name of the trait for host implementations
+        // to fill in, so rename it if "Host" is used as a regular identifier.
+        "host" => "Host_".into(),
+        s => s.to_upper_camel_case(),
+    }
+}