[go-experimental] Ensure enum deserialization checks enum values

[bkabrda]

This new unmarshalling method for enums ensures that unmarshalling fails if the value is not in the enumeration choices. As the enum type is just a type alias in go, technically any value of the base type can actually be assigned to it - this patch prevents that at least on unmarshalling.

This allows to properly deserialize oneOf members that have a common field that has specific distinct choices for each of the members. For example:

• A and B are members of oneOf and both have a foo
• For A, foo is an enum with allowed choices a, A
• For B, foo is an enum with allowed choices b, B
• The rest of the fields is potentially the same.

Having this new unmarshalling method ensures that entity with foo: b can't get deserialized as A, but only as B, hence the oneOf deserialization works properly.

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@antihax (2017/11) @bvwells (2017/12) @grokify (2018/07) @kemokemo (2018/09) @bkabrda (2019/07)

[wing328]

The change looks good to me. I'll try to consolidate the multiple import later with another PR.