Clean crypto crate and interfaces with Signature types

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Function signatures and usages were slightly inefficient and didn't match other crates because it was written a while ago.
  • Since these will soon be used by others, updates them to be what they will most likely end up as.
• Fixes make clean command because network crate was removed in previous PR
• Makes easier for Implement BLS aggregate signature verification #44 to follow intended style and functionality needed

Reference issue to close (if applicable)

Closes

Other information and links