Fix the following error:

```
warning: unexpected `cfg` condition name: `doc_cfg`
  --> rand_core/src/lib.rs:38:13
   |
38 | #![cfg_attr(doc_cfg, feature(doc_cfg))]
   |             ^^^^^^^
```

`doc_cfg` was used just to conditionally include the `doc_cfg` feature
in docs.rs builds.  That has since been standardized and changed to
`docsrs`.

Replace `doc_cfg` with `docsrs`.

See https://docs.rs/about/builds#detecting-docsrs .

Use `#[cfg(not(feature = "std"))]` to check for `std`, not
`#[cfg(not(std))]`.

Enable the `trusted_len` feature.

Implementing `TrustLen` is now unsafe.  Fix the implementation.

`rustc` complains that `error: struct MyRng is never constructed`.
Construct it, and use it.

Implementations of `FloatAsSIMD` and `BoolAsSIMD` are only used if the
`simd_support` feature is enabled.  Instead of making their definition
and several users conditional on `simd_support`, just suppress the
unused warning.

Modern versions of `rustc` warn about some elided lifetimes:

```
warning: hiding a lifetime that's elided elsewhere is confusing
   --> src/seq/mod.rs:115:27
    |
115 |     fn choose_multiple<R>(&self, rng: &mut R, amount: usize) -> SliceChooseIter<Self, Self::Item>
    |                           ^^^^^ the lifetime is elided here     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the same lifetime is hidden here
    |
    = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
    = note: `#[warn(mismatched_lifetime_syntaxes)]` on by default
help: use `'_` for type paths
    |
115 |     fn choose_multiple<R>(&self, rng: &mut R, amount: usize) -> SliceChooseIter<'_, Self, Self::Item>
    |                                                                                 +++
```

Address the warnings.

Modern `rustc` doesn't know about the `asmjs` target.  Drop it.

v1 of `actions/cache` has been deprecated.

`simd_support` is an experimental feature, which does not work any
more.  Drop it.

The cross build test checks that `Rand` can be cross built for mips.
The Rust project no longer provides binaries for that target.  Switch
to arm, as on the `master` branch.

The latest versions of `ppv-lite86` and `libc` don't compile with
`rustc` 1.36.  When testing with `rustc` 1.36, use the last compatible
version of the dependencies.