optimizing xml loading (spirit grammars)

[springmeyer]

A surprising amount of load_map() time/calls are spent with color_factory::init_from_string() (and expression::compile()).

The reason is that the grammars a being created per parse. We need to create the grammars once. We could create them globally (and perhaps leverage boost::thread_specific_ptr) but in this case it is easy to make them members of the map_parser.

The grammar getting created/destroyed (I think):

[springmeyer]

next step (before this can be closed) is to figure out how to pass grammars to the from_xml() implementations in text_placement and formatting (/cc @herm ).

[springmeyer]

progress evident: majority of time spent has now shifted from the creation of the grammar to the actual phrase_parse:

[springmeyer]

but, where expressions are not re-using the grammar is still twice a slow:

[herm]

Perhaps we could create a new data structure to handle xml processing. It could store the grammar with correct encoding settings. This data structure could be passed to all function calls and it would be easier to expose it in python. See also #1041.

[springmeyer]

@herm - that sounds good to me.