Pluggable ECC backends - Part 1

[pipermerriam]

https://www.pivotaltracker.com/n/projects/2066033

What is wrong?

The library current uses a local python implementation of the necessary ECC functions for signature creation and verification. The library coincurve provides faster implementations of these functions but at the cost of requiring a system dependency.

• Some users will value being able to do a vanilla install without futzing with system dependencies
• Some users will value speed and be willing to install the necessary system dependencies to get it.

How can it be fixed?

Implement a pluggable backend system for all of the ECC functionality. This should start with a default backend which only delegates the current ECC functionality.

Implementation Details

• Since we don't have a formal configuration spec for Py-EVM yet, lets use environment variables. for this one, lets use PYEVM_ECC_BACKEND
• Copy the import_string function from populus: https://github.com/pipermerriam/populus/blob/master/populus/utils/module_loading.py
• Implement a get_ecc_backend_class function which reads the PYEVM_ECC_BACKEND value from os.environ and imports the class.
• Implement a get_ecc_backend function which returns the instantiated ECC backend class.
• Implement a BaseECCBackend class which implements the base needed for ECC operations.
• Implement a PurePythonECCBackend class which uses the functions found at evm.utils.ecdsa.

[DavidKnott]

@pipermerriam I think I'll take a crack at this.