Add iterate method for Alphabets

[TransGirlCodes]

Addresses #231 #232

Types of changes

This PR implements the following changes: (Please tick any or all of the following that are applicable)

• [X] :sparkles: New feature (A non-breaking change which adds functionality).
• [ ] :bug: Bug fix (A non-breaking change, which fixes an issue).
• [ ] :boom: Breaking change (fix or feature that would cause existing functionality to change).

:ballot_box_with_check: Checklist

• [ ] :art: The changes implemented is consistent with the julia style guide.
• [ ] :blue_book: I have updated and added relevant docstrings, in a manner consistent with the documentation styleguide.
• [ ] :blue_book: I have added or updated relevant user and developer manuals/documentation in docs/src/.
• [ ] :ok: There are unit tests that cover the code changes I have made.
• [ ] :ok: The unit tests cover my code changes AND they pass.
• [ ] :pencil: I have added an entry to the [UNRELEASED] section of the manually curated CHANGELOG.md file for this repository.
• [ ] :ok: All changes should be compatible with the latest stable version of Julia.
• [ ] :thought_balloon: I have commented liberally for any complex pieces of internal code.

[codecov[bot]]

Codecov Report

Merging #233 (750af04) into master (fe5aabd) will decrease coverage by 0.14%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
- Coverage   89.26%   89.11%   -0.15%     
==========================================
  Files          31       31              
  Lines        2422     2426       +4     
==========================================
  Hits         2162     2162              
- Misses        260      264       +4     

Flag	Coverage Δ
unittests	89.11% <0.00%> (-0.15%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/alphabet.jl	88.23% <0.00%> (-4.36%)	:arrow_down:

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[jakobnissen]

Needs some tests, otherwise A-OK. Why not simply defer the iteration to symbols(A), though? Edit: Nevermind, for some reason, defering iteration is less effective. Weird.