[Merged by Bors] - feat(set_theory/ordinal/basic): dot notation lemmas + golf #15348
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We introduce dot notation lemmas for proving something of the form
type r < type s
ortype r ≤ type s
by providing a principal segment, an initial segment, or a relation embedding. We renametype_le
andtype_le'
totype_le_iff
andtype_le_iff'
for consistency withtype_lt_iff
(which can't be renamed totype_lt
, as this is an existing theorem abouttype (<)
).We could introduce
lift
variants of these, but I'd rather wait until #15041 is merged, at which point I can do the analogous refactor on ordinals.