Adds two functions that take a range over pairs of serialized values
and return a serialized map value.
There are 2 functions - one operating on bytes and one operating on managed_bytes.
The version with managed_bytes is used in expression.cc, used to be a local static function.
The bytes version will be used in type_json.cc in the next commit.
Signed-off-by: Jan Ciolek <jan.ciolek@scylladb.com>
e5391f1eed