Features and improvements to iterable objects

[catb0t]

• add the any, all and none Set methods, which
  necessitate a dclone that can sometimes be
  preferred over conversion to an Array

• .any, .all, and .none on Set and Array can be
  called without a block, like map

• Block: new constants, IDENTITY and LIST_IDENTITY
  which return their argument or arguments,
  respectively, to lessen runtime overhead for
  these tiny but common callables.

User code can construct these with Block.identity
and Block.list_identity.

• Array: use the IDENTITY block if a method that
  wants a block, wasn't called with one.
  This eliminates some runtime overhead for
  nested loops, and cleans up the code with
  fewer immediate anonymous subs that can
  introduce typos.

• add the Hash.linear_selection method, which
  creates a Hash that is a subset of a hash,
  given an iterable argument of keys

• linear_selection, useful on its own, is used
  to implement Set-like operations on Hashes,
  with Sets, Hashes, Arrays, and other iterables
  on the RHS.

• Now it's possible to intersect, difference,
  union and symdiff Hashes with other objects.
  These operate primarily on the Hashes' keys.

• Last, Set.difference is extended to allow a
  Hash on the RHS, for subtracting a Hash's
  keys from a Set without needing to iterate
  it twice.

[trizen]

Good work! Thanks!