Introduce overridable protocol implementations

[y86]

Depends on #67
Closes #60
Enables the solution of #61 and #62

The goal of this PR is to introduce a way for users to override default implementations from the library, In order to achieve that, several changes were introduced:

• Add options argument to Msgpax.Packer protocol #67 that allows options to be used to configure implementations;
• Refactoring of how extensions are represented: instead of having one struct Msgpax.Ext, we have 128 structs, one for each extension type;
• new Msgpax.Unpacker protocol that can be dispatched against the specific Msgpax.Ext# structs;
• new Msgpax.Ext.defimpl macro to allow users to overwrite protocol implementations without triggering warnings about module redefinition;

[y86]

These tests need to be the last one to be run so that the overwrites don't affect the other tests. Hence the async: false, as ExUnit first loads async: true tests.