Support tagging modes

[leostera]

When serializing/deserializing values of a variant type, we'd like the constructor name placement to be configurable.

• Internally tagged with a new name:

type user [@@serde {tag = "type"}]

On an variant: Use the internally tagged enum representation, with the given tag.
On a record: Serialize the record name as a field with the given key, in front of all the real fields of the struct.

• Adjacently tagged with tag and content in different fields (only for variants):

type user [@@serde {tag = "t"; content = "c"}]

• Untagged (only for variants):

type user [@@serde { untagged }]