write_aig: Normalize symmetry in multiplication

This bumps the `aig` submodule to bring in the changes from GaloisInc/aig#14,
which changes how `aig` performs multiplication to swap the order of arguments
if the first argument is a constant. This is because the algorithm that `mul`
uses to compute multiplication is biased in its order of arguments, and having
the second argument be a constant produces a network with more full adders,
which is generally more beneficial.  I have added a test case to check to see
if the test case from #1828 now produces identical AIGs regardless of the order
of arguments.

Fixes #1828.

intTests/test1828/.gitignore

@@ -0,0 +1,2 @@
+left.aig
+right.aig

intTests/test1828/test.saw

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+write_aig "left.aig" {{ \x -> x * 0x12345678 }};
+write_aig "right.aig" {{ \x -> 0x12345678 * x }};

intTests/test1828/test.sh

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+set -e
+
+# This test case uses `write_aiger` to produce AIG files for two nearly
+# identical functions, where the only difference is the order of arguments (one
+# symbolic and one concrete) in a bitvector multiplication. If the `aig` library
+# does its job correctly, these should produce identical AIG files, so check
+# this using `diff`.
+$SAW test.saw
+diff -ru left.aig right.aig