Implement modules

[c3d]

XL modules have existed in two implementations so far:

• The XL2 implementation which is hierarchical, but has no dynamic loading, no version numbers nor much self-desccription / introspection.
• The Tao3D implementation has ABI and version checking, can load modules dynamically, has support for module description, but is not hierarchical and, paradoxically, does not deal too well with static modules.

Neither implementation covers syntax statements in modules correctly.

The new implementation should be:

• Hierarchical
• Cover static modules well
• Support dynamic modules
• Support version checks, ideally using semantic versioning (which depends on issue #20)