Add Scribunto_LuaInterpreter::wrapPhpFunction

Currently, the only way for PHP to pass a function to Lua is to pass
back a function that it received from Lua. This means, for example, that
PHP cannot implement a Lua iterator function except by registering a
library holding the function or by using loadString.

This changeset adds Scribunto_LuaInterpreter::wrapPhpFunction (and
implements it for both LuaSandbox and LuaStandalone), which takes a PHP
callable and returns a Lua function wrapping it.

Note that fallback code is included so this does not depend on
I2e552799.

Change-Id: Ic0a98eec7cc17ef4b1acee032c0f42d617b998d2