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.

internal/encoding/encoding_test.go

@@ -0,0 +1,39 @@
+package encoding
+
+import "testing"
+
+func TestCellRoundTrip(t *testing.T) {
+	for letter := range byte(26) {
+		c := Cell(letter, false)
+		if IsEmpty(c) {
+			t.Errorf("Cell(%d,false) reports empty", letter)
+		}
+		if IsBlank(c) {
+			t.Errorf("Cell(%d,false) reports blank", letter)
+		}
+		if got := Letter(c); got != letter {
+			t.Errorf("Letter(Cell(%d,false)) = %d", letter, got)
+		}
+
+		b := Cell(letter, true)
+		if !IsBlank(b) {
+			t.Errorf("Cell(%d,true) not blank", letter)
+		}
+		if got := Letter(b); got != letter {
+			t.Errorf("Letter(Cell(%d,true)) = %d, want %d", letter, got, letter)
+		}
+	}
+}
+
+func TestEmpty(t *testing.T) {
+	if !IsEmpty(Empty) {
+		t.Error("IsEmpty(Empty) = false")
+	}
+	if IsEmpty(Cell(0, false)) {
+		t.Error("IsEmpty(Cell('a')) = true")
+	}
+	// 'a' (index 0) must not collide with empty.
+	if Cell(0, false) == Empty {
+		t.Error("Cell('a') collides with Empty")
+	}
+}