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avoid quadratic getToken(i) rescans in lookahead token walks #1444

Job	Run time
Grammar Choice Conflicts	38s
Gradle Check (macos-latest)	2m 48s
Gradle Check (windows-latest)	4m 27s
Gradle Check (ubuntu-latest)	2m 59s
Maven Verify (macos-latest)	7m 25s
Maven Verify (windows-latest)	5m 40s
Maven Verify (ubuntu-latest)	4m 39s
Gradle Publish (Maven Central)	0s
Build & Deploy Sphinx Docs	0s
	28m 36s