This fixes a few things so that MuPackages now unmarshal:
* Mark Module.Members as requiring "custom" decoding. This is required
because that's the first point in the tree that leverages polymorphism.
Everything "above" this unmarshals just fine (e.g., package+modules).
* As such, stop marking Package.Modules as "custom".
* Add the Kind field to the Node. Although we won't use this for type
discrimination in the same way, since Go gives us RTTI on the structs,
it is required for unmarshaling (to avoid "unrecognized fields" errors)
and it's probably handy to have around for logging, messages, etc.
* Mark Position.Line and Column with "json" annotations so that they
unmarshal correctly.
This change eliminates boilerplate decoding logic in all the different
data structures, and instead uses a new tag-directed decoding scheme.
This works a lot like the JSON deserializers, in that it recognizes the
`json:"name"` tags, except that we permit annotation of fields that
require custom deserialization, as `json:"name,custom"`. The existing
`json:"name,omitempty"` tag is recognized for optional fields.
This adds basic custom decoding for the MuPack metadata section of
the incoming JSON/YAML. Because of the type discriminated union nature
of the incoming payload, we cannot rely on the simple built-in JSON/YAML
unmarshaling behavior. Note that for the metadata section -- what is
in this checkin -- we could have, but the IL AST nodes are problematic.
(To know what kind of structure to creat requires inspecting the "kind"
field of the IL.) We will use a reflection-driven walk of the target
structure plus a weakly typed deserialized map[string]interface{}, as
is fairly customary in Go for scenarios like this (though good libaries
seem to be lacking in this area...).
This change carries over all of the metadata shapes in the MuPack
and MuIL file formats to our Go toolset. This includes creating a
proper discriminated AST type tree along with correct annotations
so that the metadata will serialize and deserialize correctly.
