core/any: remove Provider trait

* remove `impl Provider for Error`
* rename `Demand` to `Request`
* update docstrings to focus on the conceptual API provided by `Request`
* move `core::any::{request_ref, request_value}` functions into `core::error`
* move `core::any::tag`, `core::any::Request`, an `core::any::TaggedOption` into `core::error`
* replace `provide_any` feature name w/ `error_generic_member_access`
* move `core::error::request_{ref,value} tests into core::tests::error module
* update unit and doc tests

alloc/src/lib.rs

@@ -138,7 +138,6 @@
 #![feature(maybe_uninit_uninit_array_transpose)]
 #![feature(pattern)]
 #![feature(pointer_byte_offsets)]
-#![feature(provide_any)]
 #![feature(ptr_internals)]
 #![feature(ptr_metadata)]
 #![feature(ptr_sub_ptr)]

alloc/src/sync.rs

@@ -3575,7 +3575,7 @@ impl<T: core::error::Error + ?Sized> core::error::Error for Arc<T> {
         core::error::Error::source(&**self)
     }
 
-    fn provide<'a>(&'a self, req: &mut core::any::Demand<'a>) {
+    fn provide<'a>(&'a self, req: &mut core::error::Request<'a>) {
         core::error::Error::provide(&**self, req);
     }
 }