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/component-macro/tests/codegen/simple-wasi.wit

@@ -10,6 +10,12 @@ interface wasi-filesystem {
   stat: func() -> result<descriptor-stat, errno>
 }
 
+interface wall-clock {
+  record wall-clock {
+  }
+}
+
 default world wasi {
   import wasi-filesystem: self.wasi-filesystem
+  import wall-clock: self.wall-clock
 }