Rollup merge of #143774 - oli-obk:const_from, r=fee1-dead

constify `From` and `Into`

tracking issue rust-lang/rust#143773

r? ``````@fee1-dead``````

I did not mark any impls elsewhere as `const`, those can happen on their own timeframe and don't need to be part of this MVP. But if there are some core ones you think should be in there I'll happily add them, just couldn't think of any

Author: matthiaskrgr