Make DecodeBase{32,64} always return vector, not string

Base32/base64 are mechanisms for encoding binary data. That they'd
decode to a string is just bizarre. The fact that they'd do that
based on the type of input arguments even more so.

src/i2p.cpp

@@ -70,7 +70,7 @@ static Binary DecodeI2PBase64(const std::string& i2p_b64)
 {
     const std::string& std_b64 = SwapBase64(i2p_b64);
     bool invalid;
-    Binary decoded = DecodeBase64(std_b64.c_str(), &invalid);
+    Binary decoded = DecodeBase64(std_b64, &invalid);
     if (invalid) {
         throw std::runtime_error(strprintf("Cannot decode Base64: \"%s\"", i2p_b64));
     }