crypto/elliptic: support p256 curve

[markya0616]

Before submitting a pull request, please make sure the following is done:

Checklist

• [ ] Fork the repository and create your new branch from master.
• [ ] Please mention the name of community in Pull Request title.
• [ ] Git Commit Messages
  • [ ] Use the present tense (Also in Pull Request title): "Add feature" not "Added feature"
  • [ ] Use the imperative mood (Also in Pull Request title): "Move cursor to..." not "Moves cursor to..."
  • [ ] Use rebase to squash/fixup dummy/unnecessary commits into only one commit.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07 :tada:

Comparison is base (3b7f48f) 80.52% compared to head (fb7cb19) 80.59%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #254 +/- ## ========================================== + Coverage 80.52% 80.59% +0.07% ========================================== Files 143 145 +2 Lines 11212 11214 +2 ========================================== + Hits 9028 9038 +10 + Misses 1374 1370 -4 + Partials 810 806 -4 ``` | [Impacted Files](https://codecov.io/gh/getamis/alice/pull/254?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getamis) | Coverage Δ | | |---|---|---| | [crypto/elliptic/elliptic\_curve.go](https://codecov.io/gh/getamis/alice/pull/254?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getamis#diff-Y3J5cHRvL2VsbGlwdGljL2VsbGlwdGljX2N1cnZlLmdv) | `100.00% <100.00%> (ø)` | | | [crypto/elliptic/p256.go](https://codecov.io/gh/getamis/alice/pull/254?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getamis#diff-Y3J5cHRvL2VsbGlwdGljL3AyNTYuZ28=) | `100.00% <100.00%> (ø)` | | | [crypto/elliptic/secp256k1.go](https://codecov.io/gh/getamis/alice/pull/254?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getamis#diff-Y3J5cHRvL2VsbGlwdGljL3NlY3AyNTZrMS5nbw==) | `100.00% <100.00%> (ø)` | | ... and [5 files with indirect coverage changes](https://codecov.io/gh/getamis/alice/pull/254/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getamis)

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