Add options argument to Msgpax.Packer protocol

[y86]

This PR is a step in the direction of closing #60, by adding an options argument to Msgpax.Packer protocol.

With pack/2, we can introduce options to default types, such as Float, and clients can further customize their extensions behavior at runtime.

This is a breaking change, so a helper Msgpax.defiimpl macro was introduced to help in the migration and to allow further usage of the macro when considering how provide out-of-the-box overridable extensions.