perf: refactor TypeSpecifier parsing #722

Job	Run time
feature checks	4m 0s
check WASM	34s
clippy	1m 6s
test stable --no-default-features	1m 7s
test stable	1m 39s
test stable --all-features	1m 43s
test beta --no-default-features	1m 13s
test beta	1m 14s
test beta --all-features	1m 9s
test nightly --no-default-features	1m 19s
test nightly	1m 29s
test nightly --all-features	1m 32s
test 1.65 --no-default-features	5m 5s
test 1.65	4m 31s
test 1.65 --all-features	4m 27s
docs	1m 0s
fmt	20s
	33m 28s

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