Migrated execution into its own package.

Separated blocks from parser. Made Block and Action ABCs used as a
basis for their concrete subclasses. Defined a method on Blocks that
produces an Action from a Block.
This includes significant cleanup and fixes to tests.
Added tests for actions.

Includes some minor tweaks to the action module in order to support
good patching and testing.
Specified ManifestBlock.to_action conversion.
Merge branch 'refactor/execution-lib'