Reorganize module structure of cardano-coin-selection repository.

[jonathanknowles]

Context

As part of the work to create a separate coin selection and fee balancing library, we created a repository based on minimal changes to the existing cardano-wallet library.

Decision

Simplify the module hierarchy in cardano-coin-selection, so that:

• Functions and types that are only used by tests are moved into the test suite.
• Functions and types that are located in "general-purpose" modules are moved into dedicated modules, related to their purpose.

Acceptance Criteria

• There should be no functions or types defined within src that are used only for testing.

Development

Move the following modules:

• Cardano.Unsafe → Test.Unsafe.
• Data.Vector.Shuffle → Test.Vector.Shuffle.
• Data.Vector.ShuffleSpec → Test.Vector.ShuffleSpec.

QA

• Verify that there are no functions or types defined within src that are used only for testing.

[piotr-iohk]

lgtm.