Add h/v-line and box drawing support

[japhb]

Provide horizontal/vertical line- and box-drawing using the Unicode or Code Page 437 box drawing characters. The library should also correctly handle intersection and overlap (with the user able to decide between stacked overlap semantics versus same-plane conversion to proper intersection glyphs).

[japhb]

I have an initial version of this locally as a role that can be mixed into T::P::Widget, but it only handles the original cases used by rpg-ui -- simple h/v lines and boxes (in a few different line styles) that overwrite the cells under them, not handling intersection and merging yet.

I haven't pushed this one up yet because I have a feeling adding the intersection code will change the API a bit. :-/

[japhb]

I decided to push the alpha version, without the intersection code.