Issue Description:

(This may be on the order of being a bug, at least spiritually.)

The issue is not on RangeSet itself, but the extension methods on Collection and its refined protocols. This is at [CollectionExtensions.swift|https://github.com/apple/swift-se0270-range-set/blob/master/Sources/SE0270_RangeSet/CollectionExtensions.swift.] There are two versions of the removeSubranges(_🙂 method, one on RangeReplacableCollection, and one for types that conform both to that and MutableCollection.

The second version is defined using MutableCollection as the main protocol and RangeReplaceableCollection as the conditional conformance. It should be the other way around. Mechanically, either order works. But spiritually, the method fits the philosophy of RRC and the MC condition is just an opportunity for optimization. From a non-spiritual view, it also messes up the documentation. MC methods never disrupt existing collection indices, but RRC methods always do; so it'll be confusing if an index-invalidating method appears in the documentation for MC without cause.