Simplified standard library; marked redundant filters as duplicated

See #418 for background discussion: quoting the gist of it here:

> My suggestion is simple: functions that are identical save for an optional parameter (such as all functions that end with `_by`) could drop that prefix and become multi-arity functions

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> * `recurse` takes over `recurse_down` and `..`, the former is deprecated.
> * `min` and `max` take over `min_by` and `max_by`, which are deprecated.
> * `unique` takes over `unique_by`, which is deprecated.
> * `sort` takes over `sort_by`, which is deprecated.
> * `group` replaces `group_by`, which is deprecated, for consistency.
> * `paths` gains an optional argument, so that `leaf_paths` is the same as `paths(values)`. The former is deprecated. *[1]*
> * `range` allows a one parameter invocation which assumes the range to start at zero.