Research getting rid of `Number` trait

[slowli]

The Number trait in the eval crate is a crutch necessary to get conversions for native fn args working. (Without this trait, the type param T in Value<T> cannot be distinguished from other "fundamental" types, such as bool, Vec or tuples.) This trait is the only reason the eval crate depends on num-complex (and will depend on num-bigint once #42 is implemented). Unfortunately, it's not quite clear how to cleanly get rid of Number.