[GSOC 2021] Google Summer of Code 2021 project ideas

Here are our projects for Google Summer of Code 2021:

* [Speeding up string operations in mypyc](https://github.com/mypyc/mypyc/issues/801)
* [Support singledispatch efficiently](https://github.com/mypyc/mypyc/issues/802)

**Update:** These are final! You can apply (only) to one of the above projects.