Add support for encoding/decoding u128 values

[Caio Sym]

Description of feature:

stdint.rs currently declares support for uints up to 64. u128 could be easily supported but its currently not. External clients cannot manually add this support due to:

error[E0117]: only traits defined in the current crate can be implemented for primitive types

Use case:

u128 are typically used for encoding uuids. In my case, I specifically need it as iOS's IDFV is a uuid. Adding support seems pretty much free.

Implementation discussion

Change is a one liner in stdint.rs to add impl_stdint!(u128);