Rename truncate methods on integers to narrow

[joshtriplett]

This makes them consistent with widen (formerly extend).

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[jhpratt]

While I understand the reasoning, truncate feels clearer regarding the inherently lossy behavior of the method.

[rust-bors]

☔ The latest upstream changes (presumably #157398) made this pull request unmergeable. Please resolve the merge conflicts.

[Mark-Simulacrum]

+1 to what @jhpratt said. That said, r=me if libs-api is happy with this direction (looks like it needs a rebase).