Remove code duplication when deriving Deserialize for adjacently tagged enums

[Mingun]

Adjacently tagged enums with many struct variants should get most benefit from that.

[Mingun]

@dtolnay, @oli-obk, could you look at this simple optimization?

[Mingun]

@dtolnay, @oli-obk mind to merge this very simple optimization (the second commit does the job). Expanding of variant_arms variable performed twice which creates duplication. The effect especially visible when struct variants are used, because they generate a lot of code.