pgtype.Encode(Binary|Text) do not write length

To aid in composability, these methods no longer write their own length. This
is especially useful for text formatted arrays and may be useful for future
database/sql compatibility. It also makes the code a little simpler as the
types no longer have to compute their own size.

Along with this, these methods cannot encode NULL. They now return a boolean
if they are NULL. This also benefits text array encoding as numeric arrays
require NULL to be exactly `NULL` while string arrays require NULL to be
`"NULL"`.

pgtype/pgtype_test.go

@@ -60,7 +60,7 @@ type forceTextEncoder struct {
 	e pgtype.TextEncoder
 }
 
-func (f forceTextEncoder) EncodeText(w io.Writer) error {
+func (f forceTextEncoder) EncodeText(w io.Writer) (bool, error) {
 	return f.e.EncodeText(w)
 }
 
@@ -68,7 +68,7 @@ type forceBinaryEncoder struct {
 	e pgtype.BinaryEncoder
 }
 
-func (f forceBinaryEncoder) EncodeBinary(w io.Writer) error {
+func (f forceBinaryEncoder) EncodeBinary(w io.Writer) (bool, error) {
 	return f.e.EncodeBinary(w)
 }