Re-retier some DS muts, add two new mutations: Foul Stench and Ignite Blood

Ignite Blood confers immunity to flame clouds and causes any blood
spilled in the player's sight to erupt in flames and create a flame
cloud (and also makes more blood get spilled in the first place). It's
a single-level mutation available as a replacement to Hellfire in the
fire facet (and making it a tier 2 facet instead of a tier 3 one).

Foul Stench provides resistance to rotting and miasma, sickens
monsters that attack the player in melee, and may create clouds of
miasma when the player takes large amounts of damage in melee. It's a
two-level mutation, and a level of Saprovore is gained as the
first part of the facet.

Both need plenty of testing and feedback!

Conflicts:
	crawl-ref/source/enum.h
	crawl-ref/source/misc.cc
	crawl-ref/source/player.cc