[WIP] Implement subroutine opcodes

Author: Peppece

docs/contributing.rst

@@ -7,7 +7,7 @@ Setting the stage
 ~~~~~~~~~~~~~~~~~
 
 .. note::
-    If it is the first time you install py-evm on a macOS, check https://py-evm.readthedocs.io/en/latest/guides/quickstart.html#installing-on-macos
+    If it is the first time you install py-evm on macOS, check https://py-evm.readthedocs.io/en/latest/guides/quickstart.html#installing-on-macos
 
 We need to clone the Py-EVM repository. Py-EVM depends on a submodule of the common tests across all clients, so we need to clone the repo with the ``--recursive`` flag. Example: