ElGamal: add From impls; deprecate from/to_bytes

[abcalphabet]

Problem

TryFrom and From should be used instead of from_bytes and to_bytes.

As per the discussion on PR #208

Summary of Changes

Deprecates from_bytes and to_bytes in favor of try_from and into

Fixes #

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.95652% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 81.8%. Comparing base (79e316e) to head (68e3235).
Report is 30 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #246    +/-   ##
========================================
  Coverage    81.8%    81.8%            
========================================
  Files         843      846     +3     
  Lines      228586   228938   +352     
========================================
+ Hits       187124   187427   +303     
- Misses      41462    41511    +49