feat(eddsa): Added EDDSA crypto module

[hanish520]

Added EDDSA(25519 curve) crypto module to the framework. The current design for eddsa module is messy, this PR is a starting point to take this activity. Need to discuss with others.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 76.44231% with 49 lines in your changes are missing coverage. Please review.

Project coverage is 62.52%. Comparing base (d23946a) to head (8928cbc).

Files	Patch %	Lines
crypto/eddsa/eddsa.go	73.68%	18 Missing and 7 partials :warning:
crypto/keygen/keygen.go	52.77%	13 Missing and 4 partials :warning:
crypto/multisignature.go	89.18%	4 Missing :warning:
internal/testutil/testutil.go	57.14%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #112 +/- ## ========================================== + Coverage 61.67% 62.52% +0.85% ========================================== Files 77 79 +2 Lines 7433 7576 +143 ========================================== + Hits 4584 4737 +153 + Misses 2548 2537 -11 - Partials 301 302 +1 ``` | [Flag](https://app.codecov.io/gh/relab/hotstuff/pull/112/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=relab) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/relab/hotstuff/pull/112/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=relab) | `62.52% <76.44%> (+0.85%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=relab#carryforward-flags-in-the-pull-request-comment) to find out more.

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