1. Add `centipede` to `DEFAULT_ENGINES`.
2. Remove the `fuzzing_engines` attribute from `project.yaml`s if
redundant (i.e., all default engines are included in that YAML file).
3. Temporarily disable `centipede` on `c`/`c++` projects that previously
use all `DEFAULT_ENGINES` (200 projects did not specify
`fuzzing_engines` in `project.yaml`) so that we can gradually enable
centipede later them later.
These projects were tested and it was found that their use
of MSAN does not work on Ubuntu 20.04. Therefore their use of
MSAN is being disabled.
Related: #6180
* style: Clean up the repo
* Bolster the `.gitignore`
* Follow `CONTRIBUTING.md` guidelines and wrap `README.md` to 80
characters and seperate links for readability
* Remove pesky Unicode dash in `CONTRIBUTING.md`
* Remove `.DS_STORE` files
* Rename `#project.yaml#` to something logical and enable stntax
highlighting
* Updates from reviewers
* Greatly reduce `.gitignore` to only macOS `.DS_Store` and Vim
temporary files
* Apply markdown style to `docs/index.md`
* small comma grammatical change
* http -> https
Dongge Liu