Implement verify signature syscall and cleanup

[austinabell]

Summary of changes
Changes introduced in this pull request:

• Implements verify sig syscall verify signature syscall #301
  • Removes duplicate error when syscalls are called in actors
    • (I don't actually think the out of gas exit code can be hit within the actors and maybe that is intentional)
• Crypto crate cleanup
  • A lot was setup to try to match spec 1:1 and old implementation when possible, so now more closely matches impls and cleaner interface in rust
  • Signature verification now also returns result instead of just a boolean, for better error reporting

Reference issue to close (if applicable)

Closes
#301

Other information and links

[dutterbutter]

hehehe

[dutterbutter]

Misleading comment as it does not return bool?