Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/test/bip32_tests.cpp

@@ -24,7 +24,7 @@ struct TestVector {
     std::string strHexMaster;
     std::vector<TestDerivation> vDerive;
 
-    TestVector(std::string strHexMasterIn) : strHexMaster(strHexMasterIn) {}
+    explicit TestVector(std::string strHexMasterIn) : strHexMaster(strHexMasterIn) {}
 
     TestVector& operator()(std::string pub, std::string prv, unsigned int nChild) {
         vDerive.push_back(TestDerivation());