mostly non-breaking (2 tiny stylistic output things in tests).

notes (most got resolved during development)

1. list output seems to force more --- document separators, but it seems pretty sensible
2. compact list output seems missing - raised in support serializing with compact indentation of lists bourumir-wyngs/serde-saphyr#86
3. multidoc with options seems slightly awkward since no helper fn for it - asked in support options for to_string_multiple bourumir-wyngs/serde-saphyr#87
4. struggling to use readers with multiple and options, maybe i have to inline from_multiple_with_options for a from_reader_with_options or maybe they need a combinatorial from_reader_multiple_with_options..

1 is no problem really. 4. can be worked around at the cost of some internal buffering, asking upstream about 2 + 3.

otherwise it's nicer in many ways. not having to deal with singleton_map_recursive is nice.