Implement opt-in verification (closes viperproject#1187) 

Update Prusti Dev Guide - links to a wrong file (viperproject#1293) The [prusti dev guide - 3. Verification Pipeline, 3.2. Rust compilation stage](https://viperproject.github.io/prusti-dev/dev-guide/pipeline/rust.html) refers to [prusti-dev/prusti/src/lib.rs](https://github.com/viperproject/prusti-dev/blob/9ca9cd1b9bcfd9870691fa5a7a957a90987ba4af/prusti/src/lib.rs#L44), however, this mentioned code/functions have been moved to [prusti-dev/prusti/src/lib.rs](https://github.com/viperproject/prusti-dev/blob/f6850c5036c4a85c8812b46eed8ac472ca95fe25/prusti/src/callbacks.rs#L58).

document snapshot equality (viperproject#1137) 

Encode pure functions on demand. 

Rustfmt. 

Enable Rustfmt for vir-high. 

Delete accidentally commited file. 

Fix update-deps workflow 

Merge pull request viperproject#114 from frececroka/add-vir-if-else Support "if" statement in foldunfold/semantics.rs.