This change implements `mapper.Encode` "for real" (that is, in a way
that isn't a complete embarrassment). It uses the obvious reflection
trickery to encode a tagged struct and its values as a JSON-like
in-memory map and collection of keyed values.
During this, I took the opportunity to also clean up a few other things
that had been bugging me. Namely, the presence of `mapper.Object` was
always error prone, since it isn't a true "typedef" in the sence that
it carries extra RTTI. Instead of doing that, let's just use the real
`map[string]interface{}` "JSON-map-like" object type. Even better, we
no longer require resource providers to deal with the mapper
infrastructure. Instead, the `Check` function can simply return an
array of errors. It's still best practice to return field-specific errors
to facilitate better diagnostics, but it's no longer required; and I've
added `resource.NewFieldError` to eliminate the need to import mapper.
As of this change, we can also consistently emit RPC structs with `lumi`
tags, rather than `lumi` tags on the way in and `json` on the way out.
This completes pulumi/lumi#183.
db99092334