Improve documentation of the filetest run command (#1645)

* Improve output display of RunCommand

The previous use of Debug for displaying `print` and `run` results was less than clear.

* Avoid checking the types of vectors during trampoline construction

Because DataValue only understands `V128` vectors, we avoid type-checking vector values when constructing the trampoline arguments.

* Improve the documentation of the filetest `run` command

Adds an up-to-date example of how to use the `run` and `print` directives and includes an actual use of the new directives in a SIMD arithmetic filetest.

cranelift/filetests/src/function_runner.rs

@@ -189,9 +189,8 @@ impl UnboxedValues {
 
         // Store the argument values into `values_vec`.
         for ((arg, slot), param) in arguments.iter().zip(&mut values_vec).zip(&signature.params) {
-            assert_eq!(
-                arg.ty(),
-                param.value_type,
+            assert!(
+                arg.ty() == param.value_type || arg.is_vector(),
                 "argument type mismatch: {} != {}",
                 arg.ty(),
                 param.value_type