Interface for macro handlers: add information on braces around arguments

[matze-dd]

More complicated macros in extension modules can be defined on Python level and use the interface provided by Macro(..., repl=f). The handler function f() receives a token list for each declared argument. Currently, there is no information whether a mandatory argument is enclosed in {} braces. For a proper look-ahead, as in macro \xspace, this is necessary.

The enhancement would break the current interface for macro handler functions.