Fix hand-written enum variant deserializations to allow u64 discriminant

[joshtriplett]

Automatically generated enum variant deserializers allowed any integer
type as the discriminant, but the hand-written ones for specific enum
types such as Result or IpAddr only allowed types up to u32. This broke
some non-human-readable deserializers for these enums, with
deserializers that emit any integer type as a u64. Switch the visit_u32
methods to visit_u64 methods to allow discriminants to have any size up
to a u64.

[dtolnay]

Thanks! This was missed from #963.

[joshtriplett]

Thanks for the quick review and merge, and the 1.0.116 release!