Switch to Types::Serialiser booleans

[aferreira]

This change promotes type compatibility with other CPAN serialization modules like JSON.

This is another take at https://github.com/msgpack/msgpack-perl/issues/17 – see also https://github.com/msgpack/msgpack-perl/pull/34

BREAKING CHANGE: removes stringification of true() to "true" and false() to "false". As the packages Data::MessagePack::Boolean and Types::Serialiser::Boolean are aliased, it is not safe to include this controverse coertion (different modules may have different ideas on what is a useful stringification of booleans). The "bool" and "num" (0+) overloads should work as before.