Fixing problem with depdendencies of the crate

[Pzixel]

We were analyzing our depdendencies and found out that some of the crates seems to be referenced for no clear reason.

This PR does two things:

1. move insta to the dev-deps, so the downstream crates won't have it as a dependency - it is only used in a single place in tests
2. remove Cargo.lock and update gitignore accordingly to the Rust Team Code Guidelines: https://doc.rust-lang.org/nightly/cargo/faq.html#why-have-cargolock-in-version-control

It the current redaction of the guidlines it is more vague than it was before, but overall the rule of a thumb that only binary applications should be commiting the .lock file. I noticed that because moving insta to the dev dependencies left .lock file in the invalid state (they were no longer used but were listed there anyway as you can see in the commit history).

[moodysalem]

Made the change to insta dep in 0.6.5 d6876a0

[Pzixel]

Why you didn't want to just merge the PR? I think gitignore is pretty needed as well, on mac it creates a lot of extra files in the tracking system.