Add support for stored_size option in static_vector and small_vector

[Lastique]

Both static_vector and small_vector could benefit from supporting the stored_size option that is supported by the base vector container. static_vector and small_vector are usually used for small numbers of elements, and it makes sense to be able to use a smaller type to store the number of elements in the container.

Also, just as an additional idea, static_vector could automatically use a smaller size type depending on the magnitude of the specified capacity. But my main request is still the support for explicit stored_size option.

[Lastique]

BTW, there is a documentation bug:

container/include/boost/container/options.hpp

Line 403 in 3ed1c76

//! Supported options are: \c boost::container::growth_factor and \c boost::container::stored_size

here it says stored_size is supported, but it actually is not:

container/include/boost/container/options.hpp

Lines 393 to 394 in 3ed1c76

	typedef small_vector_opt< typename packed_options::growth_factor_type
	, packed_options::inplace_alignment> implementation_defined;

stored_size is also not listed as supported in the QuickBook documentation.