NodePattern: Optimize by using placeholders for post processing

This as multiple benefits:
- allows more flexible handling of future multiple match patterns.
  Handling of '...' was already using a less flexible post-processing.
- deals automatically and better with generated temporary variables
- much less argument passing to the various compiling methods
- factorizes the `#{'.type' if seq_head}` pattern that was everywhere.
- makes debugging easier with simpler & nicer generated code