Fix fetch popup

[elopez]

With the changes to the cache system in hevm, the fetch popup and related events were stubbed, they should be fixed to work properly again

echidna/lib/Echidna/UI.hs

Lines 116 to 120 in 458ae15

	-- TODO: remove and use events for this
	-- For now, return empty cache data since accessing hevm's internal cache is complex
	let c = mempty :: Map Addr (Maybe Contract)
	let s = mempty :: Map Addr (Map W256 (Maybe W256))
	writeBChan uiChannel (FetchCacheUpdated c s)