Use smaller lookup head size

Less memory and even better performance:

  Time (mean ± σ):      1.789 s ±  0.002 s    [User: 1.779 s, System: 0.010 s]
  Range (min … max):    1.786 s …  1.793 s    10 runs

src/consts.zig

@@ -19,8 +19,9 @@ pub const history = struct {
 };
 
 pub const lookup = struct {
-    pub const bits = 17;
+    pub const bits = 15;
     pub const len = 1 << bits;
+    pub const shift = 32 - bits;
 };
 
 // TODO: organize this

src/deflate.zig

@@ -410,7 +410,8 @@ test "StreamWindow slide" {
 test "struct sizes" {
     try expect(@sizeOf(Token) == 4);
     try expect(@sizeOf(Tokens) == 131_080);
-    try expect(@sizeOf(Lookup) == 393_216);
+    // print("size of Lookup {d}\n", .{@sizeOf(Lookup)});
+    try expect(@sizeOf(Lookup) == 196_608);
 }
 
 const Tokens = struct {