Add run-length-encoding exercise

[keiravillekode]

The only thing I thought might be missing is the test generator for this exercise in generators/exercises.

The generator outputs two methods

void test_empty_string(void) {
...
}

void test_empty_string(void) {
...
}

For this exercise, we might want to change the generator to include property in the test function name, e.g. test_encode_empty_string

In general, we might need to include the full description chain, e.g. test_runlength_encode_a_string__empty_string

I'm not sure how much value there is in making the generator more general - it is fairly trivial to update tests when needed.