[SMTChecker] Added support for precise modeling of external calls to this.

Modeling external calls to this, since we can trust these calls.

fixed problem with transaction data not being restored after trusted external call

update to the tests

additional tests

changelog entry

added tests for external getters of this

libsolidity/formal/SMTEncoder.cpp

@@ -2391,6 +2391,19 @@ MemberAccess const* SMTEncoder::isEmptyPush(Expression const& _expr) const
 	return nullptr;
 }
 
+bool SMTEncoder::isTrustedExternalCall(Expression const* _expr) {
+	auto memberAccess = dynamic_cast<MemberAccess const*>(_expr);
+	if (!memberAccess)
+		return false;
+
+	auto identifier = dynamic_cast<Identifier const*>(&memberAccess->expression());
+	return identifier &&
+		identifier->name() == "this" &&
+		identifier->annotation().referencedDeclaration &&
+		dynamic_cast<MagicVariableDeclaration const*>(identifier->annotation().referencedDeclaration)
+	;
+}
+
 string SMTEncoder::extraComment()
 {
 	string extra;