HAC v.0.21 -- Ada (subset) Compiler

Main improvements since v.0.2:

• Added Virtual Machine Variables, another mean for exchanging data between the HAC program and the program hosting the VM.
• SmallAda's tasking is working again in its HAC reincarnation at least, for some simple tasks.
• HAL becomes HAT (HAC Ada Toolbox), to avoid name collision with HAL = "Hardware Abstraction Layer".