Process declarations and docstrings correctly for flet+, labels+, lambda+, defun+

For now, declarations are ignored instead of ending up somewhere in
the expansion.

Docstrings should appear at the correct syntactic position in the
expansion.

Added test for docstring in defun+.
Merge pull request #5 from scymtym/wip-defun-plus

Process declarations and docstrings correctly for flet+, labels+ ...