Use a full user data for Entity references

Only moved a single script function to a member for now, will do others
in a separate commit.
Removed NPC and character parameters from NPC functions

These functions can only be used in the context of a character talking
to an NPC, so these parameters can be deduced from that context rather
than passing them explicitly all the time.

Simplifies NPC scripting.
Simplified some script function names

npc_message     -> say
npc_choice      -> ask
npc_ask_integer -> ask_number
npc_ask_string  -> ask_string
npc_trade       -> trade
Moved functions to entity members where appropriate

Some functions were skipped for now because they may need a new name
or change of behavior. Changes:

    chr_warp                        entity:warp
    chr_get_inventory               entity:inventory
    chr_inv_change                  entity:inv_change
    chr_inv_count                   entity:inv_count
    chr_get_equipment               entity:equipment
    chr_equip_slot                  entity:equip_slot
    chr_equip_item                  entity:equip_item
    chr_unequip_slot                entity:unequip_slot
    chr_unequip_item                entity:unequip_item
    chr_get_level                   entity:level
    chr_get_exp                     entity:xp
    chr_give_exp                    entity:give_xp
    chr_get_rights                  entity:rights
    chr_set_hair_style              entity:set_hair_style
    chr_get_hair_style              entity:hair_style
    chr_set_hair_color              entity:set_hair_color
    chr_get_hair_color              entity:hair_color
    chr_get_kill_count              entity:kill_count
    chr_give_special                entity:give_special
    chr_has_special                 entity:has_special
    chr_take_special                entity:take_special
    chr_set_special_recharge_speed  entity:set_special_recharge_speed
    chr_get_special_recharge_speed  entity:special_recharge_speed
    chr_set_special_mana            entity:set_special_mana
    chr_get_special_mana            entity:special_mana
    chr_kick                        entity:kick
    exp_for_level                   xp_for_level
    monster_get_id                  entity:monster_id
    monster_change_anger            entity:change_anger
    monster_drop_anger              entity:drop_anger
    monster_get_angerlist           entity:angerlist
    being_apply_status              entity:apply_status
    being_remove_status             entity:remove_status
    being_has_status                entity:has_status
    being_set_status_time           entity:set_status_time
    being_get_status_time           entity:status_time
    being_get_gender                entity:gender
    being_set_gender                entity:set_gender
    being_type                      entity:type
    being_walk                      entity:walk
    being_say                       entity:say
    being_damage                    entity:damage
    being_heal                      entity:heal
    being_get_name                  entity:name
    being_get_action                entity:action
    being_set_action                entity:set_action
    being_get_direction             entity:direction
    being_set_direction             entity:set_direction
    being_apply_attribute_modifier  entity:apply_attribute_modifier
    being_remove_attribute_modifier entity:remove_attribute_modifier
    being_set_base_attribute        entity:set_base_attribute
    being_get_modified_attribute    entity:modified_attribute
    being_get_base_attribute        entity:base_attribute
    being_set_walkmask              entity:set_walkmask
    being_get_walkmask              entity:walkmask
    being_get_mapid                 entity:mapid
    chat_message                    entity:message
    being_register                  entity:register
    chr_shake_screen                entity:shake_screen
    chr_create_text_particle        entity:show_text_particle
    -                               entity:position
    posX                            entity:x
    posY                            entity:y
    monster_get_name                monsterclass:name
    item_get_name                   itemclass:name
Dropped the & from the Lua function tables

It's not actually necessary.