implement a `StructView{T}` type

[jrevels]

Similar to MapView, there should be a StructView{T} type that allows lazy, well-typed field access via getproperty. Then, users can really nicely stage the laziness of MsgPack's decoding/copying for their use case. For example, a Vector{StructView{T}} would eagerly "tape" all structs in the buffer, whereas ArrayView{StructView{T}} would only "tape" the array containing the structs, and let the structs themselves be "taped" only upon access, etc.