add circleci config

.circleci/config.yml

+version: 2
+
+jobs:
+  build:
+    docker:
+      # The image used to build our project, build
+      # your own using the Dockerfile provided below
+      # and replace here. I put my own image here for
+      # the example.
+      - image: abronan/rust-circleci:latest
+
+  steps:
+    - checkout
+    - run: rustup nightly cargo build
+    - save_cache:
+        key: cargo-cache
+        paths:
+          - "~/.cargo"
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