Added Ed25519 tests

[melatron]

Description of change

• Added Malleability test case.
• Added zip215 tests.
• Added ed25519 sign.input tests.
• Removed TODO that is not needed.
• Fixes issue Ed25519: Improve tests #19

Type of change

• Enhancement (a non-breaking change which adds functionality)

How the change has been tested

All existing and new tests pass.

Change checklist

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[nothingismagick]

Copying @Wollac's comment from previous PR

[nothingismagick]

Please fix Clippy (even though it isn't your fault, its because of rust 1.50.0) - this is open source gardening :D

Clippy fixed here: https://github.com/iotaledger/crypto.rs/pull/52/files#diff-eb22024a611ef759fe3cbefcf751bf535d92d3154fc0e6641944cda7a82f654b

and also add a changefile:
https://github.com/iotaledger/crypto.rs/tree/dev/.changes

[nothingismagick]

For the reviewers: I have spoken with @melatron and he will setup gpg signing for his next PRs. I will sign off for this PR.

[melatron]

@Wollac About your questions from - #53 (comment)

The idea of leaving the tests there is that in Rust you write integration tests in the /tests folder and unit tests under the file for the specific functionality you are testing. For the tests that are in src/ed25519.rs they also use crate::test_utils which is not listed as a feature in the Cargo.toml file. I guess the idea is to be used only internally. Because of that I figured that these tests are more like unit tests with internal use of test_utils.

[Wollac]

Because of that I figured that these tests are more like unit tests with internal use of test_utils.

OK, fair enough. I was mainly wondering that they are kind of redundant or duplicates with the new reference test vectors. Let's leave them in then! more tests hardly ever hurt 🙂