Redesign macro calls and argument handling

[fabpot]

This is my attempt to make macros "better". It uses modern PHP features that didn't exist when I designed macros a long time ago.

The first objective is to close the gap between their behavior and the behavior of Twig callables: functions, filters, and tests.

Here are some important changes:

• Calling a macro without passing a value for an argument that has no default value is deprecated; it is currently silently passed as null.
• Passing extra positional arguments or unknown named arguments to a macro without an explicit variadic argument is deprecated; these arguments are currently silently accepted through the implicit varargs variable.
• Explicit variadic macro arguments are now supported with ...name.
• Macros are compiled as closures stored in the macro registry, instead of public generated macro_* methods.

The refactor introduces TwigMacro and MacroArgument to represent template-defined macros with an explicit signature, similar to the existing Twig callable model.