WIP: Update to 2021-04-13

[DrTobe]

I created the patches required for all nightly compiler versions up to 2021-04-13 but stopped there because I saw that the doc_spotlight feature has been removed then:

❯ cargo +nightly-2021-04-13 build
   Compiling core_io v0.1.20210325
error[E0557]: feature has been removed
 --> src/lib.rs:7:8
  |
7 |        doc_spotlight,slice_internals,maybe_uninit_ref,mem_take,specialization)]
  |        ^^^^^^^^^^^^^ feature has been removed
  |
  = note: renamed to `doc_notable_trait`

It is easy to fix this by removing that feature from lib.rs or renaming it to doc_notable_trait but this would retroactively break all previous versions:

❯ cargo +nightly-2021-04-13 build
   Compiling core_io v0.1.20210325
    Finished dev [unoptimized + debuginfo] target(s) in 0.66s

❯ cargo +nightly-2021-04-02 build
   Compiling semver v0.1.20
   Compiling rustc_version v0.1.7
   Compiling core_io v0.1.20210325
error[E0658]: `#[doc(spotlight)]` is experimental
   --> src/7d6af6751c5726d884440d4e8d462a9ee6c5efc1/mod.rs:500:1
    |
500 | #[doc(spotlight)]
    | ^^^^^^^^^^^^^^^^^
    |
    = note: see issue #45040 <https://github.com/rust-lang/rust/issues/45040> for more information
    = help: add `#![feature(doc_spotlight)]` to the crate attributes to enable

So how would you like to handle these cases?