Add unsafe member attribute

[nwin]

Rendered view

[ftxqxd]

Believe it or not, this is the third RFC that has proposed this exact feature. See #80 and #358. (The other RFCs made it unsafe to read unsafe fields, too, but they’re otherwise identical.)

I think that the fact that this feature has been proposed independently three times basically proves that this feature is wanted. (The other RFCs weren’t closed indefinitely, though, just postponed, so I think that whether or not this feature is wanted is irrelevant until after 1.0. Still, it’ll be an interesting point to bring up when the original RFC gets reopened.)

[nwin]

Oops…I'll close it for now