suggestion for simpler/more correct MacroArgs

[nschloe]

Right now, macro args are represented as

ParsedMacroArgs(
    argspec="{",
    argnlist=[...],
)

where len(argspec) == len(argnlist). My suggestion would be to slightly alter this structure to

[
    MacroArg(
        argspec="[",
        arg=#...
    ),
    MacroArg(
        argspec="{",
        arg=#...
    ),
]

This has the advantage that illegal state are irrepresentable (len(argspec) == len(argnlist) is implicit).