implement instruction coverage based plugin

This search strategy directs the symbolic execution towards "fresh"
instructions.

mythril/laser/ethereum/plugins/implementations/coverage/coverage_strategy.py

@@ -0,0 +1,34 @@
+from mythril.laser.ethereum.strategy import BasicSearchStrategy
+from mythril.laser.ethereum.state.global_state import GlobalState
+from mythril.laser.ethereum.plugins.implementations.coverage import InstructionCoveragePlugin
+
+
+class CoverageStrategy(BasicSearchStrategy):
+    """Implements a instruction coverage based search strategy
+
+    This strategy is quite simple and effective, it prioritizes the execution of instructions that have previously been
+    uncovered. Once there is no such global state left in the work list, it will resort to using the super_strategy.
+
+    This strategy is intended to be used "on top of" another one
+    """
+    def __init__(self, super_strategy: BasicSearchStrategy, instruction_coverage_plugin: InstructionCoveragePlugin):
+        self.super_strategy = super_strategy
+        self.instruction_coverage_plugin = instruction_coverage_plugin
+        BasicSearchStrategy.__init__(self, super_strategy.work_list, super_strategy.max_depth)
+
+    def get_strategic_global_state(self) -> GlobalState:
+        """
+        Returns the first uncovered global state in the work list if it exists,
+        otherwise super_strategy.get_strategic_global_state() is returned.
+        """
+        for global_state in self.work_list:
+            if not self._is_covered(global_state):
+                self.work_list.remove(global_state)
+                return global_state
+        return self.super_strategy.get_strategic_global_state()
+
+    def _is_covered(self, global_state: GlobalState) -> bool:
+        """ Checks if the instruction for the given global state is already covered"""
+        bytecode = global_state.environment.code.bytecode
+        index = global_state.mstate.pc
+        return self.instruction_coverage_plugin.is_instruction_covered(bytecode, index)