add `#[inline(always)]` to `fn infallible`

[alejandro-vaz]

change the infallible function defined on lib.rs to always be inlined

we expect this to improve performance since before it was never inlined even with the suggestion, see #604 (comment)

also, move the function to errors.rs instead

[ryux1]

I can take this. I’ll move infallible into errors.rs, mark it #[inline(always)], update its call sites/imports, and run the v2 test and lint gates.

[alejandro-vaz]

it is yours.

[alejandro-vaz]

@ryux1 how is it going??

[alejandro-vaz]

actually I just had to kill it in #624, sorry

thanks for your initiative though