Doesn't compile with the latest compiler

[ujh]

Apparently std::num::FromPrimitive was removed. The workaround described here seems broken, too. So maybe a little more code is needed to get this to work again.

Here's the compiler error:

/Users/uh/.cargo/registry/src/github.com-1ecc6299db9ec823/strenum-0.0.3/src/lib.rs:7:7: 7:30 error: unresolved import `std::num::FromPrimitive`. There is no `FromPrimitive` in `std::num`
/Users/uh/.cargo/registry/src/github.com-1ecc6299db9ec823/strenum-0.0.3/src/lib.rs:7        use std::num::FromPrimitive;
                                                                                                ^~~~~~~~~~~~~~~~~~~~~~~
/Users/uh/.cargo/registry/src/github.com-1ecc6299db9ec823/strenum-0.0.3/src/lib.rs:5:1: 31:2 note: in expansion of strenum!
/Users/uh/.cargo/registry/src/github.com-1ecc6299db9ec823/strenum-0.0.3/src/lib.rs:39:1: 41:2 note: expansion site

[iopq]

Fixed according to the answer