Generate trampolines to call exported functions instead of using libffi (#108)

* remove codegen.rs

* Add export trampolines and remove libffi

* Remove unused extern crates

lib/clif-backend/src/lib.rs

@@ -1,4 +1,3 @@
-// pub mod codegen;
 mod call;
 mod func_env;
 mod libcalls;
@@ -6,6 +5,7 @@ mod module;
 mod module_env;
 mod relocation;
 mod resolver;
+mod trampoline;
 
 use cranelift_codegen::{
     isa,