syntax: simplify `Hir::dot` constructors #2244

Job	Run time
testfull-regex-automata	9m 3s
testfull-regex	3m 43s
msrv	2m 7s
test (stable, ubuntu-latest, stable)	3m 55s
test (stable-32, ubuntu-latest, stable, i686-unknown-linux-gnu)	2m 43s
miri-regex-automata	10m 26s
rustfmt	12s
test (stable-powerpc64, ubuntu-latest, stable, powerpc64-unknown-linux-gnu)	7m 43s
testfull-regex-capi	28s
test (stable-s390x, ubuntu-latest, stable, s390x-unknown-linux-gnu)	9m 36s
test (beta, ubuntu-latest, beta)	3m 49s
testfull-regex-syntax	2m 3s
test (nightly, ubuntu-latest, nightly)	3m 49s
test (macos, macos-latest, stable)	2m 27s
test (win-msvc, windows-latest, stable)	6m 18s
test (win-gnu, windows-latest, stable-x86_64-gnu)	3m 40s
	1h 12m 2s

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