Audit clippy `pedantic` warnings

[molpopgen]

Adding #![warn(clippy::pedantic)] to lib.rs reveals 100s of new lints! While some can be ignored, others point out some valid causes for concern, like when integer casts could cause unintended wraparound behavior.

[molpopgen]

One can also say cargo clippy --no-deps -- -W clippy::pedantic to get the same warnings without modifying the source code.

[molpopgen]

Closed via #223 #222, which have to do w/integer and pointer cast issues as well as better handling some potential panics.

Most remaining pedantic lints are either documentation or very picky style issues.