Remove some outdated enums/maps, give fields a string ident.

The enumeration `mon_id` is/was used for backwards compatibility only.
Searching for `mon_id` gives several results, but in most `mon_id` is used as a variable name and does *not* refer to the enumeration. Only two places refer to the enumeration: a commented block in mapgen.cpp and the declaration of that enum.

Searching for a specific entry of that enum - `mon_zombie_fireman` (a relatively new monster type) - gives two places only:
- the definition in mtype.h as part of the mon-id enum
- its only usage in tile_id_data.h as part of the initialization of monster_names.

Searching for monster_names gives:
- its definition in tile_id_data.h
- its only usage in savegame.cpp as part of the initialization of monster_ints.
- a comment in mtype.h

Searching for monster_ints gives:
- its declaration and definition (savegame.h and savegame.cpp)
- its initialization in savegame.cpp
- some commented out usage.

Finally monster_ints is never actually read, cata_tiles does not use the `monster_names` array since 6c1df2b8b29230a643ff31b55c088fcc5131671d, so it's useless and I have removed it.

The same applies now to monster_names, and than to the enum mon_id. A similar change has been done in d28b1884226bc37becd809202697fee2718792d3.

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The last thing that has no string ident was the field definition, so I added a string ident to the field struct and made a function to look up that ident. This is used in the iuse function for placing gas clouds from explosions and for placing cigarette smoke. It also allows to erase the field_names array from tile_id_data.h as the field "name" (actual the ident) is now implicit part of the field (as is the item type id, or the monster type id).

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I also updated some documentation stuff.

The file doc/ENUMS.md is fairly useless. It contains nothing that is not already contained in the source code. In fact it is terribly outdated and people should use the search-in-files function of their preferred editor/IDE.

doc/ENUMS.md should be deleted.