Reserve from and unsafe as keywords

[axic]

And also update documentation/chanelog with all the new keywords.

• from is Support JSON ABI interface imports #1687
• unsafe is from Exception on overflow #796
• copy Reserve keywords for 0.6.0 #6040

[axic]

I would like to actually revive #6040 and reserve copy too.

[axic]

Will move the keyword fixes (and not the new additions) from here to #7954.

[chriseth]

Didn't we say unchecked instead of unsafe?

[chriseth]

Do we really want to reserve them? Reserving them would only make sense if we intend to make kind-of-breaking changes as non-breaking.

[axic]

Didn't we say unchecked instead of unsafe?

Yes, we ended up with unchecked, but for a while thought unsafe could be useful.

Do we really want to reserve them? Reserving them would only make sense if we intend to make kind-of-breaking changes as non-breaking.

I hope we can implement #1687 before the next breaking release, that's the motivation for from.

[axic]

Should also consider ascii from #5167.

[axic]

• from is not needed, because it too common of a word, and the import parser already implements it as parsing a literal
• unsafe has no clear use yet compared to unchecked
• copy is part of a very early discussion (and not the factory syntax I thought it was)
• ascii/unicode is planned for 0.7.0