Fixed a bug in UserFunction::show_arg definition

Replaced sizeof...(Tail) inside of std::enable_if<> with (sizeof...(Tail) > 0).
Without this, user-defined functions with more than two arguments did
not compile (apparently, '2' can not be converted to 'bool' at compile time,
but '1' can).
Adding vex::raw_pointer(const vex::vector<T>) function

Useful when user wants to get a pointer to a vector instead of its
current element inside a vector expression. Could be combined with
calls to addressof/dereference operators or with user-defined functions
iterating through the vector. E.g. see implementation of a primitive
n-body problem inside tests/vector_pointer.cpp