Use a connection provider rather than long-lived connections.

The previous implementation expected to work with a context object that
contained a field `conn` that represented a valid database connection.
However, this required the caller to manage the connection's lifecycle
(close, renew, etc.). Now we expect to receive a context object that
provides a `withConn` procedure or template that accepts a statement
block and provides a `conn` variable to that code block. For example:

    createRecord(db: DbContext): Record =
        # withConn must be defined for DbContext
        db.withConn:
          # conn must be injected into the statement block context
          conn.exec(sql("INSERT INTO..."))

In addition, this change provides a connection pooling mechanism
(`DbConnPool`) as a default implementation for this hypothetical
DbContext. There is also a new function `initPool` that will create an
DbConnPool instance.

Callers of this library can modify their DbContext objects to extend
DbConnPool or simply be a type alias of DbConnPool.

fiber_orm.nimble

@@ -1,6 +1,6 @@
 # Package
 
-version       = "0.3.6"
+version       = "1.0.0"
 author        = "Jonathan Bernard"
 description   = "Lightweight Postgres ORM for Nim."
 license       = "GPL-3.0"