Deprecate methods replaced with versions in std

[bluss]

Deprecate methods and functions that now have a corresponding version in std. Note that direct conflicts like flatten have already been removed instead. These "renames" we can keep deprecated until the next version after this is released.

There's a mystery stray deprecation warning (Edit: this was resolved) in the crate's own compilation, and it doesn't have a location or file/line information. We haven't gotten quite to the bottom with that, yet. I can see it's not the glob imports inside src/lib.rs, at least.

warning: use of deprecated item 'adaptors::Step': Use std .step_by() instead

Cc #223