fix iteration on large hash trees

Bug introduced by 7a8c2ff063: a tree can be deep enough that the
representation of the path to the current item can be too big to
fit into 32 bits. It will always fit in 64 bits, though.

Merge to 5.3.4
(cherry picked from commit f09d1d1ae8c438b5a0aa371fee556dfa2dd7498b)

Conflicts:
	src/racket/src/string.c
updated HISTORY

Merge to 5.3.4 release
(cherry picked from commit 71bc32927800a798c3b3f017f4dfcb7bfdff8626)