Satisfy pointer arithmetic check

[mgravell]

• satisfy infosec scanner that we're not risking pointer wierdness (although in reality, this is always UTF8; can't be hijacked)
• prefer EncodingExtensions.GetString when it is available
• switch payload to an explicit field so we can use ^^^ via pass-by-ref

There is no actual infosec here; if the inbuilt UTF8 implementation is broken, we're already hosed; there is no extension point to replace UTF8