Add solutions for part 1

projects/01/Mux.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/Mux.hdl
+
+/** 
+ * Multiplexor:
+ * out = a if sel == 0
+ *       b otherwise
+ */
+
+CHIP Mux {
+    IN a, b, sel;
+    OUT out;
+
+    PARTS:
+    Not(in=sel, out=notsel);
+    And(a=a, b=notsel, out=outa);
+    And(a=b, b=sel, out=outb);
+
+    Or(a=outa, b=outb, out=out);
+}