Add solutions for part 1

projects/02/HalfAdder.hdl

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+// This file is part of www.nand2tetris.org
+// and the book "The Elements of Computing Systems"
+// by Nisan and Schocken, MIT Press.
+// File name: projects/02/HalfAdder.hdl
+
+/**
+ * Computes the sum of two bits.
+ */
+
+CHIP HalfAdder {
+    IN a, b;    // 1-bit inputs
+    OUT sum,    // Right bit of a + b
+        carry;  // Left bit of a + b
+
+    PARTS:
+    Xor(a=a, b=b, out=sum);
+    And(a=a, b=b, out=carry);
+}