Add position_type and const_position_type to multi_type_vector.

They are the return types of the position() methods, added for
typing convenience.
Make get_numeric(), get_boolean() and get_string() more efficient.

By taking advantage of the underlying mtv's position() method, which
eliminates redundant block position lookups.
Add a way to get a reference object to an element inside multi_type_matrix.

And some associated methods that make use of such reference object.
Add methods that allow setting element value from a position object.
Add mtv's perf test file to the Visual Studio solution.
Let's not forget to add this file.
Up the version to 0.9.0.
Combine these two calls. There is no reason why they have to separate.
Fix it here too.
And here too.
Allow access to the internal multi_type_vector storage.
Revert "Allow access to the internal multi_type_vector storage."

This reverts commit 37192ea41737ea9258273f041e2ee31ee7618f2d.

Perhaps this was not a good idea.