Add solutions for part 1

tools/builtInChips/Mux4Way16.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: tools/builtIn/Mux4Way16.hdl
+
+/**
+ * 4-way 16-bit multiplexor.  
+ * out = a if sel == 00
+ *       b if sel == 01
+ *       c if sel == 10
+ *       d if sel == 11
+ */
+
+
+CHIP Mux4Way16 {
+     
+    IN a[16], b[16], c[16], d[16], sel[2];
+    OUT out[16];
+
+    BUILTIN Mux4Way16;
+}