Rollup merge of #142067 - RalfJung:abi-map-to-str, r=workingjubilee

canon_abi: make to_erased_extern_abi just a detail in formatting

I think ideally we'd avoid ever printing `CanonAbi` to users, but that needs further changes. Personally I think it's fine for Miri to use the debug printing of `CanonAbi` until we figure that out, but I think others disagree. ;)

r? ``@workingjubilee``

Author: matthiaskrgr