Add exercise parallel-letter-frequency

Fixes #1049 (#1100)

policies.md: add reference to issue #1075

Add reference to section about adding a hints file to all exercises
with difficulty 5.

* Add hints.md and generate new README

* Remove extra line from README.md

* Add hints to exercises without starter implementation

As exercises with difficulty 5 are the first exercises without starter
implementation, they should have a hints file explaining how to fix the
compiler errors that occur when an exercise doesn't have any starter
implementation.

This should be detailed in the policies doc.

* flatten-array: regenerate README

Also update description for how to generate readme as it was lacking
detail.

* changed initial reverse string stub to include argument

* change reverse string initial solution argument to match ref solution

* made the class as the package private and the removed the public access modifier

* made the class as the package private and the removed the public access modifier

* removing the extra spaces

* removed the access modifier of the class Hamming and also the methods involving it

* added the private access modifier for the methods in Hamming class

* "#1081 Removed public class modifiers"

* Changed Meetup method to private per comments on Pull Request #1082

* Made Meetup method public again as it did not pass the checks for Pull Request #1082.

* Changed MeetupTest.java to private per comments on Pull Request #1082

* Made MeetupTest public again as it failed testing.

* Made Meetup method package-private per comments on Pull Request #1082.

* Corrected access modifiers per suggestions on Pull Request #1082.

Removed package-private from Meetup method and removed private from
cycleToPrev and cycleToNext methods.

* Added private access levels back to cycleToPrev and cycleToNext.

Per feedback on pull request #1082 added the private access levels back
to these two methods.

secret-handshake: fix broken link in README.md

Change reverse method signature to follow test requirements

Update CONTRIBUTING.md

Add details of README.md to description of submodule.

Add details of how to generate README.md to section on
how to implement new exercise.

markdown: add to track

Nix public access

Ignore build.gradle when CI runs tests against changeset

* make classes and methods package private #1052

* method accessor changes

This reverts commit 58ed692a57b235d3ab3f8bb68e9375287eff8e47.

This reverts commit b14f3aef3d60abc5964b0224b3ddbd006e66d95d.

removing public access modifier from IsogramChecker class and methods

Fixes #1055 

* make classes and methods package private #1055

* changed nested class to public

* made Element<T> private and its constructor package-private

grade-school: make classes and methods package private in reference solution

trying something else

* Temporary commit to test journey test fix

* Fix journey test

Make sure run-journey-test-from-ci doesn't try to run settings.gradle as
an exercise.

* Revert "Temporary commit to test journey test fix"

This reverts commit eb0d5dd091d8f3c633fcda7742312a2c261bff8b.

maintainers.json: add @sjwarner-bp

etl: remove public access modifier from etl

clock: make classes and methods package private in reference solution