add lint infallible_try_from

[ComputerDruid]

Looks for TryFrom implementations with uninhabited error types and suggests to implement From instead.

Fixes #2144

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changelog: Added [infallible_try_from] to nursery

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[ComputerDruid]

r? @llogiq

Written at the workshop during RustWeek

[llogiq]

Small nit: We should either inline that variable (because the compiler will unify the access anyway) or use it everywhere (like one line below this).

[llogiq]

This is a good start. The code is still failing dogfood, where clippy lints itself. Looking at the suggestions, you should change the "default lint description" in the lint definition block, and use a sym instead of comparing idents with strs in line 55. Apart from that, I have only a very small nit. Once that's dealt with, I'll start the Final Comment Period for adding this lint.