Newtype, variants and unit

[MathiasKoch]

Adds ser/de for the following types:

• unit types, i.e () & struct Test;
• newtype structs, ser/de'ing the underlying value
• Variants:
  • The newtype_variant format, i.e Variant(u32)
  • The struct_variant format, i.e Variant { x: u32, y: u16 }

• Fixes tests and clippy warnings

This is just #25 rebased on master

[MathiasKoch]

Hmm.. Not sure why the CI fails here? @eldruin ?

[MathiasKoch]

Ahh...
error[E0658]: non exhaustive is an experimental feature

How should that be handled? Go back to the manual one, and exclude it from clippy checks?

[MathiasKoch]

I think it might be a matter of raising MSRV?
non_exhaustive was introduced to stable in 1.40.0

@eldruin would you be okay with me raising MSRV to 1.40.0?

[eldruin]

I would be OK with raising the MSRV to 1.40.0.