file.d: add overflow checks

[JackStouffer]

Taken over from #4712

[dlang-bot]

Thanks for your pull request, @JackStouffer!

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[wilzbach]

Also note that this only works because readImpl is completely @trusted and getcwd isn't @safe yet.
See #5928

[wilzbach]

StdUnittest

[wilzbach]

See #5932

[JackStouffer]

@wilzbach done

done

[wilzbach]

Thanks!

[adamdruppe]

Is it sane to depend on a std.experimental module in the main production branch of the library?

[wilzbach]

We already discussed this in #4712 where the consensus between everyone (including @WalterBright and @andralex) was that using the Checked wrapper is a lot better than creeping in ugly manual overflow checks.
std.experimental is experimental because of its API not because it's not tested. Using it internally within Phobos is no problem because the user won't even know it and we can still do breaking changes with its API. It's essentially the same as if we would have added our own SafeInt wrapper internally.