Remove unsafety from Trap API (#779)

* Remove unsafety from `Trap` API

This commit removes the `unsafe impl Send` for `Trap` by removing the
internal `HostRef` and leaving `HostRef` entirely as an implementation
detail of the C API.

cc #708

* Run rustfmt

crates/api/src/lib.rs

@@ -28,6 +28,6 @@ pub use crate::instance::Instance;
 pub use crate::module::Module;
 pub use crate::r#ref::{AnyRef, HostInfo, HostRef};
 pub use crate::runtime::{Config, Engine, OptLevel, Store, Strategy};
-pub use crate::trap::{FrameInfo, Trap, TrapInfo};
+pub use crate::trap::{FrameInfo, Trap};
 pub use crate::types::*;
 pub use crate::values::*;