By default, the RocksDB C library is compiled using the number of
cores on the machine. In CircleCI, this is 36 cores. Unfortunately,
that appears to completely blow out the memory usage, causing the C
compiler invocations to be killed.
This commit reduces the *entire* parallelism of the build to avoid
that.
This configures a circleci pipeline that runs fmt, lint, test, and build operations.
I changed the fmt command to --check since --overwrite is no longer supported.
The pipeline will always run on nightly builds of rust.