Add rotational-cipher exercise

[kahgoh]

rotational-cipher for Nibbly November

[github-actions]

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[keiravillekode]

Four trivial suggestions:

1. All but the first of the test cases can use xtest() instead of test(). This suggestion would also apply to nucleotide-count/nucleotide-count.spec.js
2. No need to import exp in rotational-cipher.spec.js as it is not used.
3. In proof.spec.wat, consider defining (global $A i32 (i32.const 65)) etc. like in rna-transcription/.meta/proof.ci.wat
4. Consider adding .docs/instructions.append.md

## Reserved Memory

Bytes 64-319 of the linear memory are reserved for the input string.

The input string can be modified in place if desired.

[kahgoh]

I've just pushed a commit to fix to the fork repo, although I notice the commit or changes hasn't appeared here. Can this PR be reopened?

[keiravillekode]

With extra commits, they appear when the PR is reopened. LGTM

(Once I rebased with a closed PR instead of adding extra commits, that messed things up and the PR couldn't be reopened.)

[ErikSchierboom]

@keiravillekode Could you re-review?