Use solders under the hood for keypairs and pubkeys

[kevinheavey]

Two minor breaking changes:

• The optional nacl.public.PrivateKey in Keypair.__init__ is replaced with an optional solders.keypair.Keypair
• Keypair.sign returns a solders Signature instead of nacl signing.SignedMessage

[codecov]

Codecov Report

Merging #237 (40a00a8) into master (14c220e) will increase coverage by 0.02%.
The diff coverage is 96.29%.

@@            Coverage Diff             @@
##           master     #237      +/-   ##
==========================================
+ Coverage   91.32%   91.35%   +0.02%     
==========================================
  Files          41       40       -1     
  Lines        2767     2717      -50     
==========================================
- Hits         2527     2482      -45     
+ Misses        240      235       -5