Replication SET and ENUM fields from a big-endian to a little-
endian machine (or the opposite) that are represented using
more than 1 byte (SET fields with more than 8 members or ENUM
fields with more than 256 constants) will fail to replicate
correctly when using row-based replication.

The reason is that there are no pack() or unpack() functions
for Field_set or Field_enum, which make them rely on Field::pack
and Field::unpack. These functions pack data as strings, but
since Field_set and Field_enum use integral types for
representation, the fields are stored incorrectly on big-endian
machines.

This patch adds Field_enum::pack and Field_enum::unpack
functions that store the integral value correctly in the binary
log even on big-endian machines. Since Field_set inherits from
Field_enum, it will use the same functions for packing and
unpacking the field.