Add test run to cranelift-filetests to allow executing CLIF (#890)

* Add ability to run CLIF IR using `clif-util run [-v] {file}` and add `test run` to cranelift-filetests to allow executing CLIF

This re-factors the compile/execute parts to a FunctionRunner that is shared between cranelift-filetests and clif-util. CLIF can be now be run using `clif-util run` as well as during `clif-util test` for files with a `test run` header. As before, only functions suffixed with a `run` comment are executed. The `run: fn(...) == ...` expression syntax is left for a subsequent change.

cranelift/filetests/filetests/isa/x86/run-const.clif

@@ -0,0 +1,11 @@
+test run
+
+function %test_compare_i32() -> b1 {
+ebb0:
+    v0 = iconst.i32 42
+    v1 = iconst.i32 42
+    v2 = icmp eq v0, v1
+    return v2
+}
+
+; run