pricing API is not properly namespaced

[cfergeau]

Half of the functions in pricing.h aren't namespaced. This should be done, especially since some names are really generic ("identifier"). I did this in https://github.com/cfergeau/Droplet/commit/1970b84e527c02e0668a9fb1816521d36d8a6b42 However, maybe some of these functions should be marked as static instead of being exported ? I also have the feeling that we don't need to export all the guts of this parser in a public header file (ie most of the structures in there should be opaque).