Add load_all()/dump_all() to support multidoc YAML.

[grahamgower]

Closes #239.

[codecov]

Codecov Report

Merging #335 (c57210f) into main (7db3a05) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #335   +/-   ##
=======================================
  Coverage   99.91%   99.92%           
=======================================
  Files           4        4           
  Lines        1245     1260   +15     
=======================================
+ Hits         1244     1259   +15     
  Misses          1        1           

Impacted Files	Coverage Δ
demes/demes.py	100.00% <ø> (ø)
demes/__init__.py	88.88% <100.00%> (ø)
demes/load_dump.py	100.00% <100.00%> (ø)

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[jeromekelleher]

LGTM