* Move descriptive function comments to above the function definition.
* Adopt some luadoc styles.
* Update a stale reference to a renamed function in comments.
* Fix silly and mostly inconsequential logic bug that had gone unnoticed up
till now.
* Condense a single use variable away by using a slightly more elaborate
assignment statement.
* Add a few more local aliases for global minetest.* functions.
* Replace use of local table with compostability values with a call to
`minetest.get_item_group()`.
* Define local alias for `minetest.get_item_group`.
* Remove the now unused static compostabilty values table.
Implements a composter block with crafting recipe, and some more
uncraftable blocks representing various stages of filling.
Adds a list of items that can be used with the composter and the chances
of these items adding a layer to the composter.
Implements methods to add compostable items to the composter, to update
the composter to various levels of compost and to harvest an item of
bone meal when the composter is ready.
Textures were taken from XSSheep/Nova_Wostra Pixel Perfection texture
pack.
