Deprecation of copy_lifetime: "its questionable utility" is questionable

[mzabaluev]

The unsafe utility function std::mem::copy_lifetime has been slapped with a deprecation notice:

this function may be removed in the future due to its questionable utility

Its utility, to me at least, is clear: it's a gentler hammer than std::mem::transmute for casting raw pointers into references. Now it is helpful to fix the shortcomings of raw pointer conversion functions (#20031), and if my RFC to improve those is accepted, it could be used to implement the lifetime-anchored variants.

[alexcrichton]

The function is actually currently unstable, not deprecated. We chose to conservatively not stabilize this function before alpha, but we can certainly revisit that decision!

This issue isn't particularly actionable, and I think that the stability of this function will certainly come up during the discussion of your RFC as well as #20031, so I'm going to close this for now.