Map constructor keeps a repeated key's last value; a looping chain throws - #101

The remaining known loose ends in Map and Set, in one PR.

What

• new Map with a repeated key keeps the last value. new Map([["a", 1], ["b", 2], ["a", 3]]).get("a") was 1; it is 3 now, and the key keeps its first place, as in JavaScript.
• A chain of entries that goes round throws an Error. Six places detect it and only had a comment saying "throw exception".

Why

• The Map constructor inserted with InsertionBehavior.ThrowOnExisting. That branch was an empty stub, so a repeated key was silently skipped. It inserts with OverwriteExisting now.
• A looping chain can only come from another thread changing the collection while this one reads it. In the two insert loops (Map.tryInsert, Set.AddIfNotPresent) the check had an empty body, so the loop would never have ended. The lookups and removals returned "not found".

Change

• src/generics/lib.generics.ts:
  • InsertionBehavior.ThrowOnExisting is removed; nothing else needed it. Set's constructor and add passed it and now pass None. Their behaviour is unchanged: an equal value was already left alone.
  • The six loop checks throw new Error("Map or Set changed by another thread while in use").
• tests/map_constructor_duplicates.ts (new): repeated keys in the Map constructor, with string and number keys, and repeated values in the Set constructor.

For the reviewer

• The throws are not covered by a test: I found no way to make a chain loop from a single thread.
• InsertionBehavior is not declared in lib.d.ts, so no declaration changes.

Testing

• Release, Windows x64: the whole suite passes for gc, rc, none and own, compiled and under the JIT.
• Built locally with the compiler from % of two integers of any width is an integer remainder TypeScriptCompiler#852; this change does not depend on it.
• Not run locally: debug builds, Linux, x86 and wasm.

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