Use border styles rather than frame attribute for arrays with mixed solid/dashed/dotted frame, or missing sides

Currently, when an array has lines on all sides, the mtable gets a frame attribute with the style given by the left-hand side, which can switch the style of the other sides. For example,

\begin{array}{:c|}
\hline
a\\
\hline
\end{array}

produces an array with a dashed frame all around. Now that v3 supports border styles properly, this would be better handled using the style attribute to set the border styles individually.

When an array has borders on only some of the sides, that is currently handled through an menclose element with the proper sides made solid. This loses dashed or dotted lines, and uses different spacing, so that is not ideal. Again, using the style attribute to set borders would work better.