Add solutions for part 1

projects/01/Xor.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/01/Xor.hdl
+
+/**
+ * Exclusive-or gate:
+ * out = not (a == b)
+ */
+
+CHIP Xor {
+    IN a, b;
+    OUT out;
+
+    PARTS:
+    Not(in=a, out=nota);
+    Not(in=b, out=notb);
+    And(a=a, b=notb, out=anda);
+    And(a=nota, b=b, out=andb);
+    Or(a=anda, b=andb, out=out);
+}