Duplicated optimization flags

Two optimization flags collide:

`SCHEME_PRIM_ALWAYS_ESCAPES = SCHEME_PRIM_PRODUCES_FLONUM = 8192`

I couldn't make a example where this cause a bug, because `SCHEME_PRIM_ALWAYS_ESCAPES` is used in very few places.

The solution is to increase `SCHEME_PRIM_OPT_TYPE_SHIFT` but this will (probably) cause problems in the compilers where `int` has only 16 bits.

I think that, for now, the best solution is to add a warning comment nearby.