Fixing Allocator support in Trial.Json

[glenfe]

This library looks good, and good things provoke requests like these. Here's an initial list:

• An Allocator doesn't have to be a template with a single parameter. i.e. The template parameter shouldn't be template<typename> class Allocator but instead just class Allocator. You can default it to std::allocator<char>, it doesn't matter, since you'll be rebinding as necessary.

• An Allocator doesn't have to be default constructible or stateless. All functions should accept an allocator instance, const Allocator& a. You can default this to an instance of Allocator() but users who use allocators where this is not an option, can supply the appropriate instance.

• Allocator construction shouldn't use addressof(*ptr) but instead to_address(ptr) (using addressof(*ptr) here before ptr references an object of T is undefined behavior).

• Allocators might be final so do not derive from them unconditionally. Instead of deriving from Allocator, derive from a facility like boost::empty_value<Allocator> (which will use inheritance if not final, or otherwise just store a member).

[breese]

Thank you for the report. I decided to split them into four separate issues to make it easier to track.