Remove prune flag as an external API

It's too easy to screw up handling of pruning when you use it directly. We should direct everyone to use the `squash_changes()` API instead. Let's remove the `prune` keyword from init, and add some internal-only API to enable pruning from within `squash_changes`.

It seems this needs to go into a major version bump

> It's possible that we might be able to hide away the `prune` keyword somehow and force `squash_changes` as the only approach to do that. I think we could cover both use cases that way, but I'm not sure how it would look to launch a pruning trie from inside `squash_changes()` (which uses the `prune` keyword internally right now).

_Originally posted by @carver in https://github.com/ethereum/py-trie/pull/93#issuecomment-524116152_