vis: adding a pure golang Vis()

The current Vis() and Unvis() are using the C implementation from
MTREE(8).

But that means that cgo is used, which is not always desired.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

unvis_go_test.go

@@ -0,0 +1,45 @@
+package mtree
+
+import "testing"
+
+type runeCheck func(rune) bool
+
+func TestUnvisHelpers(t *testing.T) {
+	testset := []struct {
+		R      rune
+		Check  runeCheck
+		Expect bool
+	}{
+		{'a', ishex, true},
+		{'A', ishex, true},
+		{'z', ishex, false},
+		{'Z', ishex, false},
+		{'G', ishex, false},
+		{'1', ishex, true},
+		{'0', ishex, true},
+		{'9', ishex, true},
+		{'0', isoctal, true},
+		{'3', isoctal, true},
+		{'7', isoctal, true},
+		{'9', isoctal, false},
+		{'a', isoctal, false},
+		{'z', isoctal, false},
+		{'3', isalnum, true},
+		{'a', isalnum, true},
+		{';', isalnum, false},
+		{'!', isalnum, false},
+		{' ', isalnum, false},
+		{'3', isgraph, true},
+		{'a', isgraph, true},
+		{';', isgraph, true},
+		{'!', isgraph, true},
+		{' ', isgraph, false},
+	}
+
+	for i, ts := range testset {
+		got := ts.Check(ts.R)
+		if got != ts.Expect {
+			t.Errorf("%d: %q expected: %t; got %t", i, string(ts.R), ts.Expect, got)
+		}
+	}
+}