Add variable selectors to args

[evilrobot-01]

• add variable selectors to args, allowing argument value to be resolved from a nested value within a variable
• support indexing [] within argument variable selectors
• add pallet/call query checks (discovered an unhandled error scenario when I misspelled a pallet)
• support multiple aserts of same type (list/map) to simpifly test structure which makes use of same assert type multiple times
• run asserts in parallel to speed up runs
• run queries in parallel where possible to speed up runs (provided no query arguments reference variables)
• tests
  • add assert & variable tests
  • add query test which ensures query chaining (value from one result fed into argument of next)

[evilrobot-01]

@NachoPal Have rebased this locally, but it needs a number of fixes I have already applied to #77 so perhaps you can review/approve that and then I can continue getting the PR updated/resolved?