Map exchange string format

This is a technical description of the map exchange string format, used to share map generating configurations with other users.

The outer layer of the map exchange string format includes three angle brackets on either side: ">>>" and "<<<", which must be present for Factorio to accept the string. Between those two tokens is the map exchange data, encoded using base 64 as defined by RFC 4648 (or 3548, 2535, 2045, 1421, et al.) All numerical values are stored in unsigned little-endian format. This document will use the terms "int" to refer to 4 byte numbers and "short" to refer to 2 byte numbers. Strings are Pascal-style int length prefixed strings. The format described below is valid at least for Factorio 0.14.x, other versions may use a different version scheme.