misc: code optimisations and warnings fixes

[wthrajat]

Fixes various clippy code warnings by removing unnecessary stuff to optimize the codebase.

These includes warnings about operator precedence, redundant field names in struct initialization, single-character string constant used as pattern, unnecessary casting integer literal to u8, reference immediately dereferenced by the compiler, useless conversions, unnecessary let, return statements, closures etc.

Detailed clippy logs can be found here

[SlavikBaranov]

Hey @wthrajat! Thanks for doing this work, it's really helpful!
We're preparing 0.13 release with a lot of changes and some of your changes are conflicting with our internal version.
We'll merge your PR in 0.13, I'll leave it open in the meantime.

[wthrajat]

@SlavikBaranov cool! While at it, I added some more clints and fixes and a minor rust-doc lint error URL is not a hyperlink.