basenc: add utility

basenc is a brand-new gnu core utility (added less than 3 years ago!),
which enables some more encodings.

tests/by-util/test_basenc.rs

@@ -0,0 +1,16 @@
+use crate::common::util::*;
+
+#[test]
+fn test_z85_not_padded() {
+    // The z85 crate deviates from the standard in some cases; we have to catch those
+    new_ucmd!()
+        .args(&["--z85", "-d"])
+        .pipe_in("##########")
+        .fails()
+        .stderr_only("basenc: error: invalid input");
+    new_ucmd!()
+        .args(&["--z85"])
+        .pipe_in("123")
+        .fails()
+        .stderr_only("basenc: error: invalid input (length must be multiple of 4 characters)");
+}