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/Cargo.toml

@@ -19,7 +19,6 @@ wasmtime-jit = { path = "../jit" }
 wasmparser = { version = "0.45.1", default-features = false }
 target-lexicon = { version = "0.9.0", default-features = false }
 anyhow = "1.0.19"
-thiserror = "1.0.4"
 region = "2.0.0"
 
 [dev-dependencies]