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/trampoline/func.rs

@@ -95,7 +95,6 @@ unsafe extern "C" fn stub_fn(vmctx: *mut VMContext, call_id: u32, values_vec: *m
             0
         }
         Err(trap) => {
-            let trap = trap.trap_info_unchecked();
             record_api_trap(trap);
             1
         }