Add silt node to replace stone dust

[The4codeblocks]

implements #435
^{I have to say closes #435 because "implements" is not recognized yet as a closing word}

[OgelGames]

Shouldn't this be in technic_worldgen? It's only a machine output now, but it could be made to generate in the world later, as silt is a naturally occurring material. Also concrete is in technic_worldgen.