Support complex keys in mappings

[kpodsiad]

Currently, we assume that the mapping node consists of (scalar node, node) pairs, but YAML spec allows complex keys to be used (AFAIK it could be arbitrary YAML node) in mappings.

[lbialy]

for reference, this looks like a bit more esoteric feature, I've tried to parse an example from spec with few online yaml parsers, only this one worked: https://jsonformatter.org/yaml-parser

? - Detroit Tigers
  - Chicago cubs
:
  - 2001-07-23

? [ New York Yankees,
    Atlanta Braves ]
: [ 2001-07-02, 2001-08-12,
    2001-08-14 ]