Consider `setindex` for `assoc`

[TotalVerb]

FixedSizeArrays uses setindex to mean what assoc does in this package. Personally, I find setindex more consistent. I don't have any opinion as to whether assoc should be kept or deprecated (note that setindex has opposite argument order).

Perhaps a new package can be spun off with definitions for non-mutating versions of Base functions, and packages like FunctionalCollections can extend those. In particular, it would be useful to make push, append, and the like extensible by other packages without requiring a dependency on FunctionalCollections. But that's a different issue.

[kmsquire]

Seems quite reasonable to me. This package doesn't receive much attention, so

1. please feel free to submit a PR
2. bump the PR if you don't get a quick response.

If you are interested in helping maintain/update the package, I suspect you could get commit access quite easily.