Since `mrb_to_integer` and `mrb_to_float` does not convert the object
but checks types, they are named so by historical reason. We introduced
properly named functions.

This commit obsoletes the following functions:

* mrb_to_integer()
* mrb_to_int()
* mrb_to_float()

Use `mrb_ensure_int_type()` instead for the first 2 functions. Use
`mrb_ensure_float_type()` for the last.