Prior to this CL, Type.Width != 0 was the mark
of a Type whose Width had been calculated.
As a result, dowidth always recalculated
the width of struct{}.
This, combined with the prohibition on calculating
the width of a FuncArgsStruct and the use of
struct{} as a function argument,
meant that there were circumstances in which
it was forbidden to call dowidth on a type.
This inhibits refactoring to call dowidth automatically,
rather than explicitly.
Instead add a helper method, Type.WidthCalculated,
and implement as Type.Align > 0.
Type.Width is not a good candidate for tracking
whether the width has been calculated;
0 is a value type width, and Width is subject to
too much magic value game-playing.