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.

tests/all/component_model/bindgen.rs

@@ -97,7 +97,7 @@ mod one_import {
             hit: bool,
         }
 
-        impl foo::Foo for MyImports {
+        impl foo::Host for MyImports {
             fn foo(&mut self) -> Result<()> {
                 self.hit = true;
                 Ok(())