Simplifications in language grammar

[DivyanshuSaxena]

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