Value, EncodeBinary, EncodeText, and MarshalJSON on T instead of *T

Methods defined on T are also available on *T. This change makes Value
consistent with database/sql Value implementations. It also makes Value,
EncodeBinary, and EncodeText more convenient to use because you can
pass T or *T as an argument to a query.

The MarshalJSON change is even more significant because without it
json.Marshal would generate the "%v" format instead of the implemented
MarshalJSON.

Thought this technically changes the interface, because *T will be
automatically dereferenced as needed it shouldn't be a breaking change.

See: https://github.com/jackc/pgx/issues/538 for initial discussion.

array.go

@@ -60,7 +60,7 @@ func (dst *ArrayHeader) DecodeBinary(ci *ConnInfo, src []byte) (int, error) {
 	return rp, nil
 }
 
-func (src *ArrayHeader) EncodeBinary(ci *ConnInfo, buf []byte) []byte {
+func (src ArrayHeader) EncodeBinary(ci *ConnInfo, buf []byte) []byte {
 	buf = pgio.AppendInt32(buf, int32(len(src.Dimensions)))
 
 	var containsNull int32