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.
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.