Add Unicode Arrow Characters

[stevemolitor]

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While writing an ascii-art flow chart I noticed that the unicode character I used as a down arrow caused the box-drawing line character indentation to get messed up:

That makes sense, because 0xProto does not have the BLACK DOWN POINTING TRIANGLE character (U+25BC) character I used.

I realize there are a zillion unicode characters, and a whole bunch of arrow-like characters, and it's unreasonable to expect 0xProto to support them all.

However, supporting 4 basic up / down / left / right arrows of some sort might be nice. Perhaps these:

• LEFTWARDS ARROW U+2190
• UPWARDS ARROW U+2192
• RIGHTWARDS ARROW U+2191
• DOWNWARDS ARROW U+2193

Related issue: #6