[WIP] Implement subroutine opcodes

Author: Peppece

docs/contributing.rst

@@ -6,7 +6,7 @@ Thank you for your interest in contributing! We welcome all contributions no mat
 Setting the stage
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-**Only on macOS:** if it is the first time you install Py-EVM, you will need to install LevelDB with brew before taking the following steps:
+**Only on macOS**: if it is the first time you install Py-EVM, you will need to install LevelDB with brew before taking the following steps:
 
 .. code:: sh