Left side of assignment can be a cast operation

This is done solely to explain away how assignment works outside strict
namespace blocks. Do not use this feature in actual code. This feature might be
removed in the future.

src/semantic/expr.c

@@ -1242,6 +1242,7 @@ void test_cast( struct semantic* semantic, struct expr_test* test,
    result->spec = cast->spec;
    result->complete = true;
    result->usable = true;
+   result->modifiable = operand.modifiable;
    if ( operand.folded ) {
       result->value = operand.value;
       result->folded = true;
@@ -1307,7 +1308,7 @@ void invalid_cast( struct semantic* semantic, struct cast* cast,
    str_init( &operand_type_s );
    s_present_type( &operand_type, &operand_type_s );
    s_diag( semantic, DIAG_POS_ERR, &cast->pos,
-      "%s operand cannot be cast to %s",
+      "operand (`%s`) cannot be cast to specified type (`%s`)",
        operand_type_s.value, cast_type_s.value );
    str_deinit( &cast_type_s );
    str_deinit( &operand_type_s );