Clear memory before restore.

[LukeSerne]

This fixes #1136 by unmapping everything before applying the restore operation. This will make sure that after a restore, the memory state will be exactly the same as it was when the memory's save method was called.

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• This PR only contains minor fixes.
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[chinggg]

@wtdcode @elicn Do you think mem.restore() should have the semantic to clear all memory?

[elicn]

That is a good question.
I tend to say "yes", because restore operations are usually used to overwrite an existing state with a previous one. Since we restore the entire memory map here (as opposed to restoring each memory region individually), I think this is expected.

[LukeSerne]

What is the status of this PR? I see some tests are failing on ubuntu, but I don't really see why. Personally, I think that if the tests assume the old behaviour (where a restore would not clear the memory state), they should be changed.