Bumps dart SDK to 2.12

Bumps dart SDK to 2.12
Bumps FFI to >1.0
Bumps pact native library to 0.1.1
Bumps dependencies to the latest version supporting the new SDK
Adapts code to take advantage of null safety

Version 0.1.0

This release introduces pact matching in the test phase.

pub.dev publishing

minor changes for pub.dev publishing

Initial version

Small DSL with a minimum feature set to build pacts.
Pact broker client to manage pacts.