Add some general std::vector utility functions

Added are:

* Vector(arg1,arg2,arg3,...) constructs a vector with the specified
  arguments as elements. The vector's type is derived from the
  arguments. If some of the arguments are rvalue references, they
  will be moved into place rather than copied (which can't be achieved
  using list initialization).

* Cat(vector1,vector2) returns a concatenation of the two vectors,
  efficiently moving elements when relevant.

Vector generalizes (and replaces) the Singleton function in
src/descriptor.cpp, and Cat replaces the Cat function in bech32.cpp

src/outputtype.cpp

@@ -10,6 +10,7 @@
 #include <script/sign.h>
 #include <script/signingprovider.h>
 #include <script/standard.h>
+#include <util/vector.h>
 
 #include <assert.h>
 #include <string>
@@ -65,12 +66,13 @@ CTxDestination GetDestinationForKey(const CPubKey& key, OutputType type)
 std::vector<CTxDestination> GetAllDestinationsForKey(const CPubKey& key)
 {
     PKHash keyid(key);
+    CTxDestination p2pkh{keyid};
     if (key.IsCompressed()) {
         CTxDestination segwit = WitnessV0KeyHash(keyid);
         CTxDestination p2sh = ScriptHash(GetScriptForDestination(segwit));
-        return std::vector<CTxDestination>{std::move(keyid), std::move(p2sh), std::move(segwit)};
+        return Vector(std::move(p2pkh), std::move(p2sh), std::move(segwit));
     } else {
-        return std::vector<CTxDestination>{std::move(keyid)};
+        return Vector(std::move(p2pkh));
     }
 }