Fix captoken codec, add SigCapability, move Signer to its own module

[jmcardon]

This PR moves Signer into its own module, as it isn't really used outside of the chainweb and request API.

Moreover: I am adding a SigCapability newtype, that essentially has a fixed json codec for the sake of both chainweb, as well as testing.

The changes here are mostly a refactoring, so it's covered by previous tests for signers. That said: in fixing the codec of ModuleGuard, I realize we definitely need a golden test for stable encoding. This will be done by @rsoeldner

PR checklist:

• Test coverage for the proposed changes
• PR description contains example output from repl interaction or a snippet from unit test output
• (If Relevant) Documentation has been (manually) updated at https://docs.kadena.io/pact

Additionally, please justify why you should or should not do the following:

• Benchmark regressions
  Doesn't impact perf
• Confirm replay/back compat (Ignore until core release)
• (For Kadena engineers) Run integration-tests against a Chainweb built with this version of Pact (Ignore until core release)

[rsoeldner]

We approved in a call, forgot to approve it on GH.