feat: remove unsafe fn from construction of combiners

[robjtede]

Motivation

IpCidrCombiner is not unsafe, in that contains no unsafe blocks or calls to other unsafe functions. Upholding logic requirements with unsafe is considered a misuse.

It's a shame that downstream #[forbid(unsafe)] crates can't use these constructors.

Proposal

Remove the unsafe declarations for these _unchecked functions and detail in the documentation what the logic requirements are for the arguments. (I haven't been able to figure out exactly what these requirements are from reading push since there's no comments explaining what's going on.)

Also introduce constructors that use push internally for cases where the caller cannot guarantee the logic requirements.

Interested on your thoughts on this.