[ITensors] [ENHANCEMENT] Try using ValSplit for op and related functions

[mtfishman]

Currently we rely on dynamic dispatch to dispatch to functions like op, since we convert runtime objects (tags, strings, etc.) into types like OpName to be used by dispatch. I just noticed a neat package ValSplit.jl that optimizes that code pattern by internally rewriting that kind of dispatch into a single method that uses if statements, and therefore avoids dynamic dispatch. Would be neat to see if that improves performance anywhere (though I've only seen a few examples where lookups to op showed any performance issues, and those cases could be optimized away with memoization).