Add u128 feature for complete u128 compatibility

[astocko]

Added a u128 feature with a feature gated bitflags width type for use in the macro. Enables functionality like debug formatting of flags that exceed 64 bits. #56

[alexcrichton]

This seems to only be used in the formatting pieces, and there could we just use $T instead of an explicit size? That way we could defer this decision entirely to downstream crates I think?

[astocko]

Implemented as suggested. The change does require absolute paths for custom types and type aliases.

[alexcrichton]

Oh right sorry I forgot about that :(, that's actually a pretty crucial reason for this feature.

In that case I guess we'll have to stick with an unstable feature

[astocko]

I will revert to my original PR. Any comments or suggestions?

[alexcrichton]

Looks like this may still be the requires-absolute-paths version?

[alexcrichton]

Thanks!

[dtolnay]

#61 reimplements this in a different way that neither requires absolute paths nor a cfg to opt into u128 compatibility.