Attempt at widening the arities for multimethod calls

[bshepherdson]

There's a lot of apply/RestFn/invoke etc. dynamic call machinery in Methodical's stack traces. This is an attempt to remove some of it by going up to 7 direct args for multimethod calls. (And dispatch functions.)

This hasn't removed much of the apply overhead in practice because with-meta on a function wraps it with a naive function subclass that always does a dynamic call. There are probably still some places that more dynamic calls are creeping in, but I ran out of time to dig deeper.

This may not go anywhere until I get back, but I wanted to publish this just in case.

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[alexander-yakushev]

This can be closed now as the same changes were merged in #150.