Add snippet to check if function exists.
Fix brass alloying recipe

Commit d55ecc39f954b33c17ae9a1da4aeff6382fcb790 introduced a bogus
reference to "technic:copper_ingot".
Fix craft guide for alloy cooking et al

A typo in commit d55ecc39f954b33c17ae9a1da4aeff6382fcb790 made recipes
for alloy cooking, compressing, and all other craft types sharing that
machine code, to be shown with three ingredient slots instead of the
correct one or two.
Fix fuel-fired alloy furnace

Commit ee0765804c0a21deeb2f33c22ac1a36cb0db5f43 broke the fuel-fired alloy
furnace, by removing the definition of its formspec that it requires to
set up the form upon construction.
Machine casing item

The casing is intended to be an ingredient in craft recipes for machines.
It isn't actually used in any recipes yet.  Although mainly a craft
item, it is defined as a node type, mainly to get an appropriately cubic
inventory image.  It is incidentally possible to place it as a node:
this makes some sense, although the empty machine casing isn't actually
useful as a node.
Use machine casing in most machine recipes
Consistently use cable in electric machine recipes

All electric machine recipes now include cable of the appropriate tier
as the bottom-middle ingredient, immediately below the casing ingredient.
Many LV machines were using a copper ingot in that location.
Tweak supply converter recipe

The casing is now in the center of the grid, as it is for all other
machines.
Fine gold and silver wire items

Not used in any recipes yet.
Handheld prospecting tool

The new tool will say whether a target block type is present in a
specified region, to allow for more targeted digging.  It is deliberately
quite weak, with several limitations: only stores enough charge for a
small number of shots; target can only be set by pointing at an example
node; range is limited; accuracy is less than 100%.  Some of these
limitations should probably be ameliorated, but not entirely eliminated,
in the future when we have a better idea of game balance.

The inventory image is only a placeholder.