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/bech32.cpp

@@ -3,6 +3,7 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <bech32.h>
+#include <util/vector.h>
 
 #include <assert.h>
 
@@ -26,13 +27,6 @@ const int8_t CHARSET_REV[128] = {
      1,  0,  3, 16, 11, 28, 12, 14,  6,  4,  2, -1, -1, -1, -1, -1
 };
 
-/** Concatenate two byte arrays. */
-data Cat(data x, const data& y)
-{
-    x.insert(x.end(), y.begin(), y.end());
-    return x;
-}
-
 /** This function will compute what 6 5-bit values to XOR into the last 6 input values, in order to
  *  make the checksum 0. These 6 values are packed together in a single 30-bit integer. The higher
  *  bits correspond to earlier values. */