repl: add more stats and tests; compiler: tcc fixes

compiler/tests/repl/immutable_len_fields.repl

@@ -0,0 +1,30 @@
+mut s := 'hello world'
+s.len = 0   // Error (field len immutable)
+
+mut a := []string
+a.len = 0   // Error (field len immutable)
+
+mut ints := []int
+ints.len = 0   // Error (field len immutable)
+
+println('BYE')
+===output===
+.vrepl_temp.v:2:5: cannot modify immutable field `len` (type `string`)
+declare the field with `mut:`
+struct string {
+  mut:
+	len int
+}
+.vrepl_temp.v:3:5: cannot modify immutable field `len` (type `array`)
+declare the field with `mut:`
+struct array {
+  mut:
+	len int
+}
+.vrepl_temp.v:4:8: cannot modify immutable field `len` (type `array`)
+declare the field with `mut:`
+struct array {
+  mut:
+	len int
+}
+BYE