[0.79] Fixed crucible NBT synchronization and metal amount/ratio display in gui

There were a couple of annoying bugs preventing the crucible from working correctly.

1) Alloys inside them were not properly saving themselves to NBT.
2) The crucible was not actually writing its alloy to NBT.

As a side note, crucibles/metals are writing their amounts as floats but they are always integers, fixing this would cause compatibility problems, so I left it in.