v2.1.0: More `Map`-conformance + optimisation

Fixes:

• .delete now no longer throws when targeting non-existent paths

New features, to better match behaviour of Map:

• .delete now returns a Boolean
• .set now returns a self-reference, so you can chain .set calls

Improved performance by about 20× for .get and .has, 8× for .set, 3× for .delete, and 1.3× for iterators, by converting recursive algorithms to iterative.