Support for the load statement in the compiler

[mosser]

Porting the load statement from the previous compiler to the new architecture.

• Exposing a "partial compilation chain" providing a list of actions out of a file name;
• Load statements reified as a LoadFile action
• When interpreting the action, triggering the partial chain to get actions, and then using the execution engine to enrich the already existing Unit
• Recording already visited files to avoid loops or double loading.