APIs for ES6 DataView #202

N-API currently has APIs for ArrayBuffer and TypedArray classes, but not the DataView class. The DataView class is specified by ES6 as another kind of view over an ArrayBuffer.

While it could be possible to extend (and rename?) napi_create_typedarray() and napi_get_typedarray_info() to also support DataView objects, I think the combination would be too confusing, because a DataView doesn't have a type, and measures length in terms of bytes instead of elements. So, probably there should be separate APIs:

• napi_is_dataview()
• napi_create_dataview()
• napi_get_dataview_info()