Fix v swap, assignment operators and constructors.

Assignment operators didn't work correctly because operatorT()
was not really a const function and this_type (used in those functions)
was not decalared. This resulted in compile error while trying to
assign v<> of different types.

Additionally v<> constructor taking const T& as parameter is not
needed. Using it would mean assigning some value to underlaing val
in v<>, but first call to get() would run constructor (which would
potentially change the value).