Lua ustring implementation

This is a reimplementation of Lua's string library with support for
UTF-8.

The entire ustring library is implemented in pure Lua. PHP callbacks are
also available for overrides: in LuaSandbox these are used for almost
all functions, while in LuaStandalone they are used only for the pattern
matching. Also, ustring.upper and ustring.lower are overridden using
mw.language's .uc and .lc if available.

It also includes a bunch of unit tests.

Note that if you download the normalization tests, they may fail under
LuaSandbox if you have PHP's intl extension installed and libicu on your
system is too old.

Change-Id: Ie76fdf8d3a85d0a3d2a41b0d3b7afe433f247af0
Merge "Lua ustring implementation"