Make ToBuiltin handle any kind of built-in list

[effectfully]

PlutusTx.Builtins.Class has the following:

instance ToBuiltin [BuiltinData] (BuiltinList BuiltinData) where
    {-# INLINABLE toBuiltin #-}
    toBuiltin []     = mkNilData unitval
    toBuiltin (d:ds) = mkCons d (toBuiltin ds)

instance ToBuiltin [(BuiltinData, BuiltinData)] (BuiltinList (BuiltinPair BuiltinData BuiltinData)) where
    {-# INLINABLE toBuiltin #-}
    toBuiltin []     = mkNilPairData unitval
    toBuiltin (d:ds) = mkCons (toBuiltin d) (toBuiltin ds)

We should be able to handle any kind of built-in list, since we don't really need those mkNilData and mkNilPairData builtins as we can simply embed the empty list constants directly.

[effectfully]

ToBuiltin is now called ToOpaque.

[effectfully]

Was fixed by #6347.