Remove HostRef as a reexport from wasmtime (#794)

This continues #788 and literally removes the type from the public API
of the `wasmtime` crate, making it inaccessible to the outside world.
Now it's purely an implementation detail, yay!

crates/api/src/lib.rs

@@ -26,7 +26,7 @@ pub use crate::callable::Callable;
 pub use crate::externals::*;
 pub use crate::instance::Instance;
 pub use crate::module::Module;
-pub use crate::r#ref::{AnyRef, HostInfo, HostRef};
+pub use crate::r#ref::AnyRef;
 pub use crate::runtime::{Config, Engine, OptLevel, Store, Strategy};
 pub use crate::trap::{FrameInfo, Trap};
 pub use crate::types::*;