Fix clippy issues

[dacut]

This PR fixes a few issues identified via cargo clippy --all-features. I interspersed a few #[allow(clippy::...)] bits where it seemed like readability was more important than the changes Clippy was suggesting or where performance might be affected (notably cases where &Cow<> is taken as a parameter instead of Cow<>, seemingly intentionally).

Might hold off on this one until the serialization PR (#490) lands; I can fix the conflicts that will result afterward.

[codecov-commenter]

Codecov Report

Merging #494 (864b733) into master (221b57d) will increase coverage by 0.03%.
The diff coverage is 42.85%.

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@@            Coverage Diff             @@
##           master     #494      +/-   ##
==========================================
+ Coverage   61.25%   61.28%   +0.03%     
==========================================
  Files          32       32              
  Lines       15654    15649       -5     
==========================================
+ Hits         9589     9591       +2     
+ Misses       6065     6058       -7     

Flag	Coverage Δ
unittests	61.28% <42.85%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
examples/custom_entities.rs	0.00% <0.00%> (ø)
examples/nested_readers.rs	0.00% <0.00%> (ø)
src/de/mod.rs	56.44% <ø> (+0.23%)	⬆️
src/de/simple_type.rs	93.65% <ø> (ø)
src/events/mod.rs	71.64% <0.00%> (ø)
src/reader/ns_reader.rs	63.71% <ø> (ø)
src/reader/slice_reader.rs	100.00% <ø> (ø)
src/utils.rs	73.22% <0.00%> (+0.57%)	⬆️
src/events/attributes.rs	93.56% <50.00%> (ø)
src/escapei.rs	13.24% <100.00%> (-0.06%)	⬇️
... and 6 more

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[dralley]

@dacut Could you rebase?

[dacut]

Ah, missed that the other issue was resolved. Will rebase and resolve.

[dacut]

Ok, rebased. cargo clippy is still (amazingly) showing clean, despite all the changes made to clippy in the meantime. (A lot of my projects with clippy checks broke recently.)