Implement Scrabble move generator (DAWG) with English and Russian rules

A Go library that returns every legal play ranked by score and scores or
validates plays, using the Appel-Jacobson DAWG algorithm over
github.com/iliadenisov/dafsa v1.1.0.

- DAWG move generation (across / down / both), full tournament scoring with a
  per-tile breakdown; public Solver: GenerateMoves (ranked), ScorePlay,
  ValidatePlay.
- Rulesets: English Scrabble, Russian Scrabble, Эрудит (parameterizable Ruleset).
- cmd/builddict (build the DAWG from the dictionaries submodule), cmd/stress
  (self-play benchmark), selfplay engine; brute-force test oracle.
- A GADDAG was implemented, benchmarked and removed (the DAWG was smaller and
  faster for a scoring solver); see RESULTS.md and ALGORITHM.md.

rack/rack_test.go

@@ -0,0 +1,51 @@
+package rack
+
+import "testing"
+
+func TestRackBasics(t *testing.T) {
+	r := New(26)
+	if !r.Empty() || r.Total() != 0 {
+		t.Fatal("new rack not empty")
+	}
+
+	r.Add(0) // a
+	r.Add(0)
+	r.Add(2) // c
+	r.AddBlank()
+
+	if r.Count(0) != 2 {
+		t.Errorf("Count(a) = %d, want 2", r.Count(0))
+	}
+	if !r.Has(2) || r.Has(1) {
+		t.Errorf("Has c=%v b=%v, want true,false", r.Has(2), r.Has(1))
+	}
+	if r.Blanks() != 1 {
+		t.Errorf("Blanks = %d, want 1", r.Blanks())
+	}
+	if r.Total() != 4 {
+		t.Errorf("Total = %d, want 4", r.Total())
+	}
+
+	r.Remove(0)
+	if r.Count(0) != 1 {
+		t.Errorf("after Remove, Count(a) = %d, want 1", r.Count(0))
+	}
+	r.RemoveBlank()
+	if r.Blanks() != 0 {
+		t.Errorf("after RemoveBlank, Blanks = %d, want 0", r.Blanks())
+	}
+}
+
+func TestRackCloneIndependent(t *testing.T) {
+	r := New(26)
+	r.Add(0)
+	cp := r.Clone()
+	cp.Add(0)
+	cp.AddBlank()
+	if r.Count(0) != 1 || r.Blanks() != 0 {
+		t.Errorf("mutating clone changed original: a=%d blanks=%d", r.Count(0), r.Blanks())
+	}
+	if cp.Count(0) != 2 || cp.Blanks() != 1 {
+		t.Errorf("clone wrong: a=%d blanks=%d", cp.Count(0), cp.Blanks())
+	}
+}