Add corner asphalt, wood, terracotta shingles
thanks to a code contribution from Bas080.

Old raillike nodes will still work but you can't obtain them anymore except
via /giveme.  Whatever you have of them can be converted to the new nodes by
placing them in your crafting grid - 1:1 conversion.

Craft the slope shape with the standard dirt/sand/coal/plastic,
terracotta, and wood/sticks recipes.  Craft the "outer" corner by
placing two slopes side by side (yields 2).  Craft the "inner" corner by
placing three slopes in an "L" shape (yields 3).  The new nodes can be
converted back and forth between inner/outer corners and slopes without loss.

In the future I will add an autoplace algorithm to make the corner nodes
automatically appear as needed.