Rename engine's Universal type to Backend

The Universal type was essentially a builder of engines that's given a
compiler backend and creates an Engine with .engine() method. The name
was not clear.

examples/instance.rs

@@ -15,7 +15,7 @@
 //! Ready?
 
 use wasmer::{imports, wat2wasm, FunctionEnv, Instance, Module, Store, TypedFunction};
-use wasmer_compiler::Universal;
+use wasmer_compiler::Backend;
 use wasmer_compiler_cranelift::Cranelift;
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
@@ -39,7 +39,7 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     // Note that we don't need to specify the engine/compiler if we want to use
     // the default provided by Wasmer.
     // You can use `Store::default()` for that.
-    let mut store = Store::new_with_engine(&Universal::new(Cranelift::default()).engine());
+    let mut store = Store::new_with_engine(&Backend::new(Cranelift::default()).engine());
     let mut env = FunctionEnv::new(&mut store, ());
 
     println!("Compiling module...");