Future ideas for Provider API support

[shepmaster]

provide(deref) shorthand for strings et. al. (String => &str)

For example:

struct Error {
    #[provide(deref)]
    name: String,
}

Disable providing for opaque errors

For example, make this test pass:

#[snafu(provide(u8 => 1))]
struct InternalError;

struct OpaqueError(InternalError);

assert!(opaque_error.request_value::<u8>().is_none())

Skip providing the value when chaining

For example, make this test pass (ideally improving the syntax as well):

struct ThisProvidesAnI32;

#[snafu(provide(ref, chain, ThisProvidesAnInteger => chained))]
struct Error {
    chained: ThisProvidesAnInteger,
}

assert!(error.request_ref::<ThisProvidesAnInteger>().is_none());    
assert!(error.request_value::<i32>().is_some());

Change ErrorCompat::backtrace to use/prefer the Provider API

[constituent]

Currently data provided by the wrapped source error will be automatically available on the wrapping error. However I have a use case (something like source itself, that wrapping error and wrapped error should provide different value), that I'd like to disable this default behavior.

I'm using provide(priority, ... to ensure that data comes from the wrapping error, but I'd like a more explicit feature, maybe provide(no_chain, ...