Helper function that creates a temporary file
Add classes that mimic STL vector but use mmap internally
Add convenience function grow_and_map() to typed_mmap
Great index map/multimap refactoring

The many different index map and multimap implementations are very confusing.
This refactoring moves low-level functionality into helper classes, renames
a lot of classes and removes some. We still need a lot of documentation to
make usage clear.