Fix duplicated impl links

[GuillaumeGomez]

Fixes #78701.

The problem is that the blanket impl has the same ID as the other impl, except that we don't derive IDs when we generate the sidebar. We now do.

r? @notriddle

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  IMAGE: x86_64-gnu-tools
##[endgroup]
From https://github.com/rust-lang/rust
 * branch              master     -> FETCH_HEAD
Searching for toolstate changes between 93230281562cd6b1b45eff070c473e3be20d9e72 and 426ea0cd5b848232a55bbd9fef4343807dab597c
Rustdoc was updated
##[group]Run src/ci/scripts/verify-channel.sh
src/ci/scripts/verify-channel.sh
shell: /usr/bin/bash --noprofile --norc -e -o pipefail {0}
env:
---
................... (50/60)
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/checkout/src/test/rustdoc-gui/toggle-click-deadspace.goml An exception occured: Failed to launch the browser process!
Inconsistency detected by ld.so: dl-tls.c: 493: _dl_allocate_tls_init: Assertion `listp->slotinfo[cnt].gen <= GL(dl_tls_generation)' failed!


TROUBLESHOOTING: https://github.com/puppeteer/puppeteer/blob/master/docs/troubleshooting.md
== STACKTRACE ==
Error
Error
    at innerRunTestCode (/node-v14.4.0-linux-x64/lib/node_modules/browser-ui-test/src/index.js:468:16)
    at runMicrotasks (<anonymous>)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)

[GuillaumeGomez]

Spurious failure. I'll really need to try to prevent it somehow...

[notriddle]

@bors r+

[bors]

📌 Commit 9b8a6b9 has been approved by notriddle