Use incremental builds for the release profile
Use incremental builds for the release profile

Dramatically improves the speed of rebuilding release (i.e. not from
scratch). Little to no difference in runtime on awcy.

https://beta.arewecompressedyet.com/?job=codegen-units-incremental%402020-06-17T17%3A26%3A41.471Z&job=codegen-units%402020-06-17T14%3A13%3A14.505Z

before:
  release profile: full build time
    real    0m51.344s
    user    6m47.160s
  release profile: touch src/lib.rs
    real    0m18.483s
    user    1m29.580s

after:
  release profile: full build time
    real    0m48.754s
    user    6m53.001s
  release profile: touch src/lib.rs
    real    0m3.177s
    user    0m2.793s

xiph/rav1e,xiph/rav1e