Merge branch Davi0kProgramsThings:fix/refactoring into branch bitfinexcom:master. (#238)

# Description
<!--- Describe your changes in detail -->
PR includes some global refactoring in preparation for the v3.0.0 stable release.

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
-

## Related Issue
<!--- If suggesting a new feature or change, please discuss it in an issue first -->
<!--- If fixing a bug, there should be an issue describing it with steps to reproduce -->
<!--- Please link to the issue here: -->
PR fixes the following issue: -

## Type of change
<!-- Select the most suitable choice and remove the others from the checklist -->

- [X] Bug fix (non-breaking change which fixes an issue);

# Checklist:

- [X] I've done a self-review of my code;
- [X] I've made corresponding changes to the documentation;
- [X] I've made sure my changes generate no warnings;
- [X] mypy returns no errors when run on the root package;
<!-- If you use pre-commit hooks you can always check off the following tasks -->
- [X] I've run black to format my code;
- [X] I've run isort to format my code's import statements;
- [X] flake8 reports no errors when run on the entire code base;

bfxapi/rest/_bfx_rest_interface.py

@@ -0,0 +1,20 @@
+from typing import Optional
+
+from bfxapi.rest._interfaces import (
+    RestAuthEndpoints,
+    RestMerchantEndpoints,
+    RestPublicEndpoints,
+)
+
+
+class BfxRestInterface:
+    def __init__(
+        self, host: str, api_key: Optional[str] = None, api_secret: Optional[str] = None
+    ):
+        self.auth = RestAuthEndpoints(host=host, api_key=api_key, api_secret=api_secret)
+
+        self.merchant = RestMerchantEndpoints(
+            host=host, api_key=api_key, api_secret=api_secret
+        )
+
+        self.public = RestPublicEndpoints(host=host)