Added IEnumerable<T> interface to NativeHashSet

[sarkahn]

No description provided.

[jacksondunstan]

@sarkahn Thanks for submitting this PR! I agree that implementinng IEnumerable<T> in NativeHashSet<T> is an improvement. I'd just like to see some changes before merging:

• This project supports Unity 2018.1 which didn't yet have non-experimental support for C# 6 and its expression-bodied members. To maintain this support, please implement these as regular methods.
• Please conform to the codebase's style by always adding curly braces to blocks, only using a single consecutive blank line, avoiding var, always adding access specifiers, putting curly braces on their own lines, omitting blank lines before the end of a block, and naming fields like m_FieldName.
• Please remove all commented-out code
• Please add unit tests to thoroughly exercise all functionality you added
• Please make NativeHashSet<T>.Enumerator match NativeLinkedList<T>.Enumerator, such as by implementing IEquatable<Enumerator>, not performing safety checks in the constructor, keeping a copy of the whole NativeHashSet<T> rather than its contents.
• Please add xml-doc to all struct contents: fields, methods, properties, other structs, etc. For methods and properties, include the safety requirements (e.g. read or write) and the runtime complexity (e.g. O(n)).
• Please only perform safety checks when necessary (e.g. not in the Enumerator constructor)
• Please replace the second if in Enumerator.MoveNext, which is redundant, with an else
• Please reuse NativeMultiHashSetIterator in Enumerator to reduce duplication

I'll be happy to take another look once these changes are in. Thanks again!