[m-rev.] for post-commit review: move checked conversions to int to the library

[Zoltan Somogyi]

For review by Julien, since he has this paged in. The only things
worth looking at are the new comments on the moved predicates,
and whether my evaluation of the relative performance of the
old and new implementations of int64.to_int is correct.

Zoltan.
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