function.is_protected: Improve heuristic

[montyly]

function.is_protected is heavily used by the inbuilt-analyses and many detectors:

slither/slither/core/declarations/function.py

Lines 1133 to 1149 in 1b15a40

	def is_protected(self):
	"""
	Determine if the function is protected using a check on msg.sender
	Only detects if msg.sender is directly used in a condition
	For example, it wont work for:
	address a = msg.sender
	require(a == owner)
	Returns
	(bool)
	"""
	if self.is_constructor:
	return True
	conditional_vars = self.all_conditional_solidity_variables_read(include_loop=False)
	args_vars = self.all_solidity_variables_used_as_args()
	return SolidityVariableComposed('msg.sender') in conditional_vars + args_vars

This function was introduced ~1,5 year ago, and gives too many false positives and false negatives (ex: require(msg.sender == tx.origin), #399). We need to improve it. some ideas:

• Use all_slithir_operations, and check for direct comparison with msg.sender
• Find a way to capture correctly msg.sender that are used in a call/modifier. The data dependency can't be used here, as it relies on is_protected. An alternative would be to compute first the data dependency without the protected information, and recompute it a second time. This would give better results but will require some refactoring.
• We should make it a property and memoize it