Differentiate the various types of giant

[copperwater]

• Hill giant: this is the "unspecial" giant, it should perhaps have its stats (base level, difficulty etc) swapped with stone giant.
• Stone giant: if not carrying a boulder and standing on diggable floor not next to water, may pry a boulder up out of the floor, creating a pit on that space (that they don't fall into). This expends their mspec_used and sets it to a high amount - 80 to 100 or more turns? Also, may spawn with an extra boulder.
• Fire giant: passive fire attack, an active fire attack would be nice but none of the attack types really fit (AT_TUCH seems weird for a giant, and physical types like AT_CLAW seem like they ought to do physical damage, but don't if paired with AD_FIRE). Maybe it can be fire breath?
• Frost giant: same as fire giant, but with cold.
• Storm giants: have an active lightning attack (AT_MAGC AD_ELEC?) This should be limited by mspec_used so that they can't use it every move, and should not be flavored as casting a spell because giants aren't supposed to be that learned.

All giants may need their difficulties bumped.

See YANI #1678, #3158