Update `TODO` after mruby3.0.

TODO

-Things to do (Things that are not done yet)
-
-* special variables ($1,$2..)
-* super in aliased methods
-
-Things to improve (Done but things to fix)
-
-* Make additions as they are noticed.

TODO.md

+Thing to Do in the future
+===
+
+# After mruby 3.0
+
+* replace `fp_fmt.c` by `float_format` (https://github.com/dhylands/format-float.git)
+* multi-precision integer
+* WORD_BOXING: Pack some floats in `mrb_value`
+* NAN_BOXING: Allow `MRB_INT64` along with NaN boxing
+* keyword arguments a la Ruby3.0 (using `OP_SENDVK`)
+* parser and code generator independent from `mrb_state` (mmruby?)
+
+# Things to do (Things that are not done yet)
+
+* `begin ... end while cond` to behave as CRuby
+* special variables ($1,$2..)
+* super in aliased methods