Use vectors instead of dicts for storing matches by using debrujin indexing for slot variables

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In each rule, we know how many slot variables there are. We can assign each one of them an index. A vector holding the matches is much more performant than an immutabledict or a linkedlist Working on it in https://github.com/JuliaSymbolics/Metatheory.jl/pull/77

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Less performant in the end