*: support invoking methods by offset

Allow to invoke methods by offset:
1. Every invoked contract must have manifest.
2. Check arguments count on invocation.
3. Change AppCall to a regular syscall.
4. Add test suite for `System.Contract.Call`.

pkg/compiler/analysis.go

@@ -16,7 +16,6 @@ var (
 	goBuiltins = []string{"len", "append", "panic"}
 	// Custom builtin utility functions.
 	customBuiltins = []string{
-		"AppCall",
 		"FromAddress", "Equals",
 		"ToBool", "ToByteArray", "ToInteger",
 	}