This PR changes the encoding logic for values such that Variants with fields are encoded as valid Daml.

This works by adding a special case for the encoding of a Variant v whose value is a Record r, where r and v's package id and module name match and where r's entity name is equal to v's plus . and a suffix.

Closes #14723

Spun off as #15153:

Note that there's still a problematic case:

Variant(variantId = V, constructor = C, value = Unit)

This could represent a value of variant type V with constructor C, where C takes no arguments, e.g.

data V = A Int | B Text | C

var : V
var = C

or a value of variant type V a with constructor C a, where a ~ (), e.g.

data V a = A Int | B Text | C a

var : V ()
var = C ()

There is no way to tell these apart without the type information at hand. We could make the value field of Variant optional, leaving value unset for the former case and set as Unit for the latter, but it's currently commented as Required in https://github.com/digital-asset/daml/blob/main/ledger-api/grpc-definitions/com/daml/ledger/api/v1/value.proto#L146-L148