Deprecate `io` module

[kt3k]

APIs in io are based on the legacy Reader/Writer interfaces. We should deprecate io before stabilizing std.

part of #3489

[iuioiua]

Are web-streams-equivalent solutions on par with the performance of Reader/Writer solutions? If not, users can expect a performance hit due to this deprecation. Last time I checked, a web streams version of x/r2d2 would be slower than the Reader/Writer, though not majorly.