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
