std/io: add Discard

std/io/io.jule

@@ -139,6 +139,23 @@ trait Stream {
 	Closer
 }
 
+struct discard{}
+
+impl Writer for discard {
+	fn Write(mut self, buf: []byte)!: (n: int) {
+		ret len(buf)
+	}
+}
+
+impl StrWriter for discard {
+	fn WriteStr(mut self, s: str)!: (n: int) {
+		ret len(s)
+	}
+}
+
+// A [Writer] on which all Write calls succeed without doing anything.
+static mut Discard = discard{}
+
 // Copy copies from src to dst until either EOF is reached
 // on src or an error occurs. It returns the number of bytes
 // copied and the first error encountered while copying, if any.