chore: Move small numbers into their own file

src/small_number.ts

@@ -0,0 +1,49 @@
+import { type Options, SizedType } from "./mod.ts";
+
+type U2Number = 0 | 1 | 2 | 3;
+type U4Number = U2Number | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15;
+
+export class U2 extends SizedType<U2Number> {
+  constructor() {
+    super(1, 1);
+  }
+
+  readPacked(dt: DataView, options: Options = { byteOffset: 0 }): U2Number {
+    const v = dt.getUint8(options.byteOffset) & 0b11;
+    super.incrementOffset(options);
+    return v as U2Number;
+  }
+
+  writePacked(
+    value: U2Number,
+    dt: DataView,
+    options: Options = { byteOffset: 0 },
+  ): void {
+    dt.setUint8(options.byteOffset, value & 0b11);
+    super.incrementOffset(options);
+  }
+}
+
+export class U4 extends SizedType<U4Number> {
+  constructor() {
+    super(1, 1);
+  }
+
+  readPacked(dt: DataView, options: Options = { byteOffset: 0 }): U4Number {
+    const v = dt.getUint8(options.byteOffset) & 0b1111;
+    super.incrementOffset(options);
+    return v as U4Number;
+  }
+
+  writePacked(
+    value: U4Number,
+    dt: DataView,
+    options: Options = { byteOffset: 0 },
+  ): void {
+    dt.setUint8(options.byteOffset, value & 0b1111);
+    super.incrementOffset(options);
+  }
+}
+
+export const u2: U2 = new U2();
+export const u4: U4 = new U4();