feat(postgres): Bootstrap Postgres persistence implementation

Adds the initial finito-postgres crate with type definitions for the
tables and initial functions to interact with persisted FSMs.

This is far from feature complete at this commit.

finito-postgres/src/error.rs

@@ -0,0 +1,8 @@
+//! This module defines error types and conversions for issue that can
+//! occur while dealing with persisted state machines.
+
+use std::result;
+
+pub type Result<T> = result::Result<T, Error>;
+
+pub enum Error { SomeError }