Bring ast_to_src up to date

Bring ast_to_src up to date with the main repository at
http://git.eclipse.org/c/koneki/org.eclipse.koneki.metalua.git/
Multiple enhancements to ast_to_src

Processes unary minus correctly, produces valid code for string literal
method calls and IIFEs, processes label, goto, and the `Stat node,
escapes invalid identifiers.