TryDemangleError does not impl std::error::Error

[athre0z]

This makes try_demangle inconvenient to use when working with type-erased error handling.

[krtab]

I believe this is because Error is currently still in std. (cf rust-lang/rust#103765)

I see two options:

1. Add a feature to rustc-demangle to rely on std, and hence get access to Error
2. Add a feature to rustc-demangle to rely on nightly and hence get acces to #![feature(error_in_core)]