Implements a map of unsigned 64-bit keys into strings. The map is optimized for creating it once, and then reading many times. The struct `Builder` is used to build the map, and the struct `Map` is used for lookups. The special property of the implementation is that it encodes all the data needed for the lookup in a single sequence of bytes. This makes it rather interesting for dynamic loading of data that can then be placed in an operating system's read only memory. The internal structure requires no decoding when it is loaded (say from a file).