Remove std.mem.toSliceConst, have toSlice return the `const`-ness of its argument

[daurnimator]

@daurnimator

It might make sense to remove toSliceConst entirely and have the return value have the const-ness of the argument?

@andrewrk

I agree with this. I would even go so far as to say it's worth having the parameter accept a var type in general to accommodate this.

Originally posted in #3972

[andrewrk]

I opened a related proposal #4000. So far I'm leaning towards accepting both.

[andrewrk]

Done in 5b26128. Deleting deprecated functions will happen naturally, or as part of #1629.