allow more kinds of fields to take 0 space. fixes #8156

also initialize datatype->instance eagerly, so you don't need to check
for that everywhere. this also makes it easier to change the logic
of which types are singletons.
Fix indentation in sort docs
Add -n option to sphinx in release-candidate

SPHINXOPTS="-W -n" means:
  -W turn all warnings into errors
  -n check cross-references

Docs are finally scoped and cross-referenced correctly, so let's keep it
that way.
use correct predicate for which types are singletons
Merge branch 'jb/size0stuff'
Remove empty base/comparison.jl
Merge pull request #8169 from nalimilan/comparison

Remove empty base/comparison.jl
fix #8156
doc typo
Merge pull request #8173 from trevor/patch-1

doc typo
Merge pull request #8167 from JuliaLang/jq/dates

Change nowutc() to now(Dates.UTC()) for a better API with TimeZones.jl
fix doc typo
Merge pull request #8181 from rfourquet/docs-promote_type

fix doc typo
Stab at dates test hanging