[Python-ideas] Add list.join() please

[Jamesie Pic]

Oops, fixing my last example:

readable = mapjoin(
    'hello',
    f'__{name}__',
    sep='\n',
    # key=format_record, could be used here
)

Signature would be like (illustrating defaults): mapjoin(*args,
sep='\n', key=str)