Add temporary ui test of 'multiple applicable items' error

Add test of getting backtrace from an anyhow source

Add test involving backtrace from a Box<dyn Error>

    error: usage of wildcard import
      --> src/aserror.rs:45:9
       |
    45 |     use super::*;
       |         ^^^^^^^^ help: try: `super::{Error, UnwindSafe}`
       |
       = note: `-D clippy::wildcard-imports` implied by `-D clippy::pedantic`
       = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wildcard_imports

Add Sealed supertrait to private AsDynError trait

The aserror module is already private to thiserror, with AsDynError
being re-exported in thiserror::private, so as long as Sealed is not
also re-exported there, it remains inaccessible outside the crate.

Currently fails with:

    error[E0034]: multiple applicable items in scope
       --> tests/test_backtrace.rs:165:13
        |
    165 |             x: std::io::Error,
        |             ^ multiple `provide` found
        |
        = note: candidate #1 is defined in an impl of the trait `Provider` for the type `E`
        = note: candidate #2 is defined in an impl of the trait `std::error::Error` for the type `std::io::Error`
    help: disambiguate the associated function for candidate #1
        |
    165 |             Provider::provide(&x, Error): std::io::Error,
        |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    help: disambiguate the associated function for candidate #2
        |
    165 |             std::error::Error::provide(&x, Error): std::io::Error,
        |             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This reverts commit f924c25.

Fix "Multiple applicable provide methods in scope"

Pull in Provider impl from anyhow 1.0.65