compiler: simplify interface lowering

This commit simplifies the IR a little bit: instead of calling
pseudo-functions runtime.interfaceImplements and
runtime.interfaceMethod, real declared functions are being called that
are then defined in the interface lowering pass. This should simplify
the interaction between various transformation passes. It also reduces
the number of lines of code, which is generally a good thing.

transform/wasm-abi.go

@@ -38,6 +38,12 @@ func ExternalInt64AsPtr(mod llvm.Module) error {
 			// coroutine lowering.
 			continue
 		}
+		if !fn.GetStringAttributeAtIndex(-1, "tinygo-methods").IsNil() {
+			// These are internal functions (interface method call, interface
+			// type assert) that will be lowered by the interface lowering pass.
+			// Don't transform them.
+			continue
+		}
 
 		hasInt64 := false
 		paramTypes := []llvm.Type{}