Add test coverage for struct member offsets in addition to alignment
Implement Marshal.OffsetOf
Implement IntPtr and UIntPtr's constructors and To(U)Int(32|64)
Fix some incorrect uses of JSIL.Host.warning.
Struct array fields tagged with the new JSPackedArray attribute are backed by a JS TypedArray instead of a native JS array, reducing their memory usage and making it possible to pin them.