grains: add signatures to stub solution

[rbasso]

Since we changed grains to expect a Maybe in #197, we had two users that opened issues about not being able to compile it: #305 and #430

The test suite seems correct, but users have a hard time trying to infer the correct types based on test suite.

This PR just add the expected type signatures to the stub solution.

[rbasso]

As time passes I get more convinced that all exercises should obey the following:

• Have a stub solution
• Compile and run the tests with the stub solution
• Fail the first test with the stub solution

Edit:

• Have variable names in the stub implementation and a body of undefined.

Not sure yet about:

• Have a HINTS.md file