Implement Dexter entrypoint serialization

[sskeirik]

Fixes: #192 Fixes: #193

This PR implements:

• abstract entrypoint parameters for the Dexter contract
• a weak abstract entrypoint parameter validation method
• a method to serialize them into concrete Dexter contract parameters

[sskeirik]

@ehildenb @hjorthjort This PR is ready for final review. The previous review comments have been addressed.