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/wasm.rs

@@ -7,9 +7,10 @@
 
 use super::{
     AnyRef, Callable, Engine, ExportType, Extern, ExternType, Func, FuncType, Global, GlobalType,
-    HostInfo, HostRef, ImportType, Instance, Limits, Memory, MemoryType, Module, Store, Table,
-    TableType, Trap, Val, ValType,
+    ImportType, Instance, Limits, Memory, MemoryType, Module, Store, Table, TableType, Trap, Val,
+    ValType,
 };
+use crate::r#ref::{HostInfo, HostRef};
 use std::cell::RefCell;
 use std::rc::Rc;
 use std::{mem, ptr, slice};