Simplifications in language grammar

Since the language grammar is directly converted into a JSON file and then used, it is better to simplify the language at some point.

Listing the possible simplifications here:

[ ] Drop `Selector` from `Match` enum (since we only have Contexts now)
[ ] No need for `Vec<Match>` in `Policy` struct
[ ] Upgrade `Context` to `Policy` struct directly instead of indirection via `Match`