Use "unsafe" where appropriate

[edsko]

Many of the foreign exports are not declared "unsafe", which means that are less efficient than they could be. They should be marked as "unsafe" where appropriate (which I think is more or less everywhere).

This is low priority, just recorded it here so that it's not forgotten.

[kvanbere]

Bump, do you think you could implement this, or would you accept a PR?

[portnov]

Hi.
I will accept a pull request, but currently I have no time enough to implement this myself.

[edsko]

Be aware that any foreign calls that are not expected to return immediately should not be marked as unsafe, as doing so will make them uninterruptible.