Fix or silence some more clippy warnings (#954)

* fix some clippy warnings

* apply rustfmt

* add ink_engine to PURELY_STD_CRATES list in GitLab CI
23 jobs for master in 9 minutes and 28 seconds (queued for 10 seconds)
Status Job ID Name Coverage
  Check
passed #1167536
linux-docker
check-std

00:02:48

passed #1167537
linux-docker
check-wasm

00:02:04

 
  Workspace
passed #1167538
linux-docker
build-std

00:00:58

passed #1167539
linux-docker
build-wasm

00:01:40

passed #1167544
linux-docker
clippy-std

00:01:30

canceled #1167558
linux-docker
clippy-wasm

00:00:19

passed #1167543
linux-docker
codecov

00:06:18

passed #1167541
linux-docker
docs

00:01:23

passed #1167546
linux-docker
fmt

00:01:23

failed #1167542
linux-docker
spellcheck

00:00:48

passed #1167540
linux-docker
test

00:04:23

failed #1167545
linux-docker
clippy-wasm

00:00:45

 
  Examples
passed #1167550
linux-docker
examples-clippy-std

00:01:17

skipped #1167551
linux-docker
examples-clippy-wasm
skipped #1167552
linux-docker
examples-contract-build
skipped #1167553
linux-docker
examples-contract-build-delegator
skipped #1167554
linux-docker
examples-docs
skipped #1167549
linux-docker
examples-fmt
passed #1167547
linux-docker
examples-test

00:01:35

passed #1167548
linux-docker
examples-test-experimental-engine

00:01:21

 
  Ink Waterfall
skipped #1167555
kubernetes-parity-build allowed to fail
ink-waterfall
 
  Fuzz
skipped #1167556
linux-docker
fuzz-tests
 
  Publish
passed #1167557
kubernetes-parity-build
publish-docs

00:00:36

 
Name Stage Failure
failed
spellcheck Workspace
     |
186 | - the optionally set callable's selector `s`
| ^^^^^^^^^^
| Possible spelling mistake found.

[2021-10-10T10:11:11Z WARN cargo_spellcheck::suggestion] Unable to get terminal size. Using default: 80
[2021-10-10T10:11:11Z INFO cargo_spellcheck::action] ❌ /builds/parity/ink/crates/lang/ir/src/ir/item_impl/callable.rs : 1
Cleaning up file based variables
ERROR: Job failed: exit code 1