Add mw.loadData, for static data loading

People sometimes want to load large tables of constant data from a
module. Using require(), this has to be reparsed every time, which can
be slow.

mw.loadData() will load the just data once, and return a table with a
metatable cleverly designed to give read-only access to the loaded data.

Change-Id: Icec192bdbe6cfca7902fd5cb5d0e217bd8399637