Concrete syntax changes

- Change the concrete syntax for implication to `==>` instead of `=>` that we use now. This has better readability and less ambiguity.

- Change the symbol for storage update to `:=` instead of `=>` that we use now. This make the syntax consistent with the ones for constructors. I also find this more intuitive.

The issue names no files or tests. Start by locating the concrete-syntax handling for implication and storage updates, then trace any parser or syntax tests covering the current symbols. Done means implication uses `=>` and storage update uses `:=` consistently, with affected tests or examples updated.

Written by the indexing model from the issue text.