builtin: add panic_optional_not_set and use it in cgen for fncall()? in main

vlib/builtin/builtin.c.v

@@ -47,6 +47,10 @@ fn panic_debug(line_no int, file string, mod string, fn_name string, s string) {
 	}
 }
 
+pub fn panic_optional_not_set(s string) {
+	panic('optional not set ($s)')
+}
+
 // panic prints a nice error message, then exits the process with exit code of 1.
 // It also shows a backtrace on most platforms.
 pub fn panic(s string) {

vlib/v/gen/c/cgen.v

@@ -5570,7 +5570,7 @@ fn (mut g Gen) or_block(var_name string, or_block ast.OrExpr, return_type table.
 				paline, pafile, pamod, pafn := g.panic_debug_info(or_block.pos)
 				g.writeln('panic_debug($paline, tos3("$pafile"), tos3("$pamod"), tos3("$pafn"), *${cvar_name}.err.msg );')
 			} else {
-				g.writeln('\tv_panic(_STR("optional not set (%.*s\\000)", 2, ${cvar_name}.err.msg));')
+				g.writeln('\tpanic_optional_not_set(*${cvar_name}.err.msg);')
 			}
 		} else if !isnil(g.fn_decl) && g.fn_decl.is_test {
 			g.gen_failing_error_propagation_for_test_fn(or_block, cvar_name)

vlib/v/markused/markused.v

@@ -137,6 +137,7 @@ pub fn mark_used(mut the_table table.Table, pref &pref.Preferences, ast_files []
 	if pref.is_debug {
 		all_fn_root_names << 'panic_debug'
 	}
+	all_fn_root_names << 'panic_optional_not_set'
 	if pref.is_test {
 		all_fn_root_names << 'main.cb_assertion_ok'
 		all_fn_root_names << 'main.cb_assertion_failed'