a priority queue block
peek
ParseString already asserts rule interfaces to maps
dedupe block emits unique messages, resolves #370
Merge pull request #376 from jacqui/master

new block: dedupe
first draft of cache block
chenged the in route to
Merge pull request #364 from mikedewar/queue

a priority queue block
Merge pull request #378 from mikedewar/cache

first draft of cache block