re2: add Set::Size

Among other uses, exposing RE2::Set::Size allows writing a function that
takes a Set and a string and reports whether the string matches every
regexp in the Set. Without Size, that function can't know how many
matches to expect.

Change-Id: Ia161a87f20d031be11a9df54b981631800af1528
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63570
Reviewed-by: Alan Donovan <adonovan@google.com>