Revisit dynamic dispatch based on types

[c3d]

Dynamic dispatch is currently somewhat implemented both in the compiler and interpreter.

However, given recent evolutions of the language, notably with respect to pattern matching, this code needs to be seriously revisited.

Key changes:

• Support for type inheritance / implicit conversions (two passes)
• Support for metaboxes (issue Implement metabox notation #10) in patterns
• Smarter dispatch in maps for common cases, e.g. contiguous values (ideally, turn that to "switch" statements in LLVM IR and let the switch optimizer do its magic).