Spirit: Add support for std::function

[mcraveiro]

At present its not possible to declare an attribute of type std::function anywhere in a diagram. It won’t really be possible to do so for entities and values because boost serialisation will always be a problem. Having said that we should make sure the parser is able to parse these expressions and worry about serialisation separately.

Examples:

std::function<void(int)> 
std::function<void()>
std::function<void(const Foo&, int)> 

This story was originally backlogged here [1].

Links:

[1] https://github.com/DomainDrivenConsulting/dogen/blob/master/doc/agile/product_backlog.org#add-support-for-stdfunction