Address clippy warnings

[sergey-melnychuk]

This PR is just a suggestion how to address all existing clippy warnings.

Each commit fixes a single category of clippy warnings and removes respective allow clause:

• add impl Default for ... or #[derive(Default)] for pub struct ... with pub fn new() -> Self
• turn impl Into<A> for B into impl From<B> for A when it makes sense
• push allow(module_inception) down from top-level lib.rs to specific modules
• add simple type alias WhenThen for non-trivial type signature
• remove irrelevant clippy allows (that are not violated in the code really)

The cargo clippy run using the toolchain below does not show any warnings:

stable-x86_64-apple-darwin (default)
rustc 1.57.0 (f1edd0429 2021-11-29)

[Dandandan]

Thanks @sergey-melnychuk !