std.Io: delete CountingWriter

[andrewrk]

Upgrade Guide:

• If you were discarding the bytes, use std.Io.Writer.Discarding, which has a count.
• If you were allocating the bytes, use std.Io.Writer.Allocating, since you can check how much was allocated.
• If you were writing to a fixed buffer, use std.Io.Writer.fixed, and then check the end position.
• Otherwise, try not to create an entire node in the stream graph solely for counting bytes. It's very disruptive to optimal buffering.

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Note: the "random patches from another branch" commits are there in case someone wants to recover that code which was written earlier but bitrotted.