MAP contains concatenation of string(key)-value pairs.
Keys *must* be non-empty, unique and length-first bytewise ascending ordered.

    MAP [STR(KEY0) || ITEM0 || STR(KEY1) || ITEM1 || ... ] EOC

Hint: Encoding code for known format can be ordered itself to emit
values in an already properly sorted way.

SET is emulated by using MAPs with NIL values. That gives only 1-byte
overhead for each element, but reuses already existing code.

Example representations:

MAP[foo: LIST["bar"]] | 09 C3666F6F 08 C3626172 00 00
SET[sig, dh]          | 09 C26468 01 C3736967 01 00