Bump upper version bounds to allow base-4.20

Bump the upper version bounds on `base` in the various `what4` packages, and
bump the `aig` submodule to bring in the changes from
https://github.com/GaloisInc/aig/pull/18.

dependencies/aig

@@ -1 +1 @@
-Subproject commit 372d533621b24ffdc7e911b92f66ddb0fdc361a9
+Subproject commit f8d96d733865791f2a950a46477c359f33c29c01

what4-abc/what4-abc.cabal

@@ -17,7 +17,7 @@ Description:
 
 library
   build-depends:
-    base >= 4.7 && < 4.20,
+    base >= 4.7 && < 4.21,
     aig,
     abcBridge >= 0.11,
     bv-sized >= 1.0.0,

what4-blt/what4-blt.cabal

@@ -26,7 +26,7 @@ common bldflags
 library
   import: bldflags
   build-depends:
-    base >= 4.7 && < 4.20,
+    base >= 4.7 && < 4.21,
     blt >= 0.12.1,
     containers,
     what4 >= 0.4,

what4-transition-system/what4-transition-system.cabal

@@ -13,7 +13,7 @@ build-type:    Simple
 common dependencies
   build-depends:
     , ansi-wl-pprint       ^>=0.6
-    , base                 >=4.12 && <4.20
+    , base                 >=4.12 && <4.21
     , bytestring
     , containers           ^>=0.6
     , io-streams