Fix #25 reported by @liammachado .

In `SimpleExprBuilder.ConstantBV()` the check of the absolute value
was off by one.

src/ExprBuilderTests/SimpleExprBuilderTests/ConstantsSimpleBuilder.cs

@@ -161,6 +161,7 @@ private void _PositiveBV(int decimalValue, int width, string expectedString)
         [TestCase(-5, 4, "11bv4")]
         [TestCase(-11, 32, "4294967285bv32")]
         [TestCase(0, 4, "0bv4")]
+        [TestCase(-8, 4, "8bv4")]
         public void NegativeBV(int decimalValue, int width, string expectedString)
         {
             _NegativeBV(decimalValue, width, expectedString);

src/Symbooglix/Expr/SimpleExprBuilder.cs

@@ -113,14 +113,14 @@ public LiteralExpr ConstantBV(BigInteger decimalValue, int bitWidth)
                 //
                 // The rule is basically this:
                 //
-                // decimal_rep_for_bits = (2^m - x) mod (2^m)
+                // decimal_rep_for_bits = (2^m - |x|) mod (2^m)
 
                 if (bitWidth <=1)
                     throw new ArgumentException("Decimal value cannot be represented in the requested number of bits");
 
                 var abs = BigInteger.Abs(decimalValue);
 
-                if (abs >= BigInteger.Pow(2, bitWidth -1))
+                if (abs > BigInteger.Pow(2, bitWidth -1))
                     throw new ArgumentException("Decimal value cannot be represented in the requested number of bits");
 
                 var result = ( twoToPowerOfBits - abs );