Rename float configuration option names.

- `MRB_WITHOUT_FLOAT` => `MRB_NO_FLOAT`
- `MRB_USE_FLOAT` => `MRB_USE_FLOAT32`

The former is to use `USE_XXX` naming convention. The latter is to make
sure `float` is 32bit float and not floating point number in general.

src/array.c

@@ -825,7 +825,7 @@ aget_index(mrb_state *mrb, mrb_value index)
   if (mrb_fixnum_p(index)) {
     return mrb_fixnum(index);
   }
-#ifndef MRB_WITHOUT_FLOAT
+#ifndef MRB_NO_FLOAT
   else if (mrb_float_p(index)) {
     return (mrb_int)mrb_float(index);
   }