static grammars
 - construct on first use
 - allows for faster map loading and unloading of xml loading tree
 - modified expression and transform grammars to not take args/not crash
 - simplifies interfaces, allows fast parsing from python without passing grammar instance
all static grammar usage const + xml_tree cleanups
cleanup + rename svg_transform parser
add TODO comments for grammars that are not stateless - refs #2231
simplify code
Merge pull request #2231 from mapnik/fast-static-grammars

static grammars