travis nightly builds are failing due to missing links in the docs

[erickt]

#706 is failing with what I think is a failure unrelated to the PR. Instead, it appears that #691 added a check for broken doc links to travis, which is now failing due to these errors: https://travis-ci.org/rust-random/rand/jobs/484131698#L1450-L2200, like:

warning: `[impls::next_u32_via_fill]` cannot be resolved, ignoring it...
   --> /home/travis/build/rust-random/rand/rand_core/src/lib.rs:144:24
    |
144 |     /// [`fill_bytes`][impls::next_u32_via_fill].
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^ cannot be resolved, ignoring
    |
    = help: to escape `[` and `]` characters, just add '\' before them like `\[` or `\]`

[dhardy]

Those warnings are incorrect — if you build the doc, they actually work. Why we still get them I don't know.

The error messages at the end appeared recently in another PR. I didn't realise they were also on master. 😒

The Android build has been failing for a couple of weeks, reason unknown. 😕

[dhardy]

deadlinks/cargo-deadlinks#36

[dhardy]

#711 takes care of most of this; rust-lang/rust#57892 the rest.
#701 documents the Android issue.