rust: make evmc-client thread safety in HostContext

Refine this module make it more like go-binding implementation.

Author: CaptainVincent

bindings/rust/evmc-client/Cargo.toml

@@ -14,4 +14,5 @@ edition = "2018"
 [dependencies]
 evmc-sys = { path = "../evmc-sys", version = "7.4.0" }
 evmc-vm = { path = "../evmc-vm", version = "7.4.0" }
-libloading = "0.5"
+libloading = "0.5"
+lazy_static = "1.4.0"