Fixed ToVariant/FromVariant derive for generic types with bounds

[Bromeon]

Fixes part of #957; but there's a separate problem with where clauses (see description).

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try

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[Bromeon]

bors try

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try

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[Bromeon]

Great, clippy stable conflicts with clippy nightly. Real good experience there 🙄 clippy nightly causes so much extra maintenance effort that I'm seriously thinking of ditching that CI job...

I'll revert the 2nd commit.

[Bromeon]

bors try

[bors[bot]]

try

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[Bromeon]

I'll merge this so the remaining problems (where clause) can be fixed in a separate PR.

bors r+

[bors[bot]]

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