Restrict zero and one to Number types. Default to floating points.

There is some room for choosing the return type of e.g. `zero(Real)` and right now the default is `Int`. By using floating points instead the `InexactError` in #4796 could have been avoided. This pr also restricts `zero` and `one` such that `zero/one(Any)` is no longer defined. It is possible that this change will break some code with type inference problems, but maybe that is a good.
Why do we have `one(Ptr{Int})`? If I need a zero pointer I use C_NULL, but I can't see any reason to have a one pointer.