Cache Brew packages for MacOS build on CircleCI

We use a hardcoded path to the brew package cache. In case it ever changes, there is a step above that prints what it should be.

.circleci/config.yml

@@ -4,7 +4,17 @@ commands:
   brew-install:
     description: "Brew install MacOS dependencies (or restore from cache)"
     steps:
+      - run: brew --cache
+      - restore_cache:
+          name: Restore Brew package cache
+          keys:
+            - brew-packages-{{ checksum "BrewDeps.txt" }}
       - run: make install-dependencies
+      - save_cache:
+          name: Save Brew package cache
+          key: brew-packages-{{ checksum "BrewDeps.txt" }}
+          paths:
+            - /Users/distiller/Library/Caches/Homebrew
   build-and-test:
     steps:
       - run: make --keep-going --jobs 16 CXXFLAGS_EXTRA="-Werror"