Give Humans exploration healing (elliptic)

Humans have long been thought of as rather bland. The intent is for
them to be a vanilla 'baseline species', but that excites very few
players in practice.

Let's move them even closer to baseline by moving their XP apt down
to the default, and give them Meteorans' exploration healing in
exchange. Humans are natural explorers, after all!

This might encourage some fun new playstyles. Numbers, as ever,
subject to change. (elliptic thought we should double explore healing,
but I want to try baseline healing first.)

crawl-ref/docs/crawl_manual.rst

@@ -1673,10 +1673,10 @@ for, and/or add a relatively complex mechanic (or change in mechanic) to
 gameplay.
 
 Humans (Hu)
-  Humans tend to be hardworking and industrious, and learn new things quickly.
-  The Human species is the most versatile of all the species available to
-  players. Humans advance quickly in levels and have equal abilities in most
-  skills.
+  Humans are natural explorers. As they uncover new spaces in the dungeon,
+  they are refreshed and invigorated, rapidly healing and recovering magic.
+  They are also the most versatile of all species - having balanced aptitudes
+  for all skills lets them adapt to use whatever they find.
 
 Kobolds (Ko)
   Kobolds are small, mysterious creatures of unknown origin. They are well

crawl-ref/source/dat/descript/mutations.txt

@@ -785,7 +785,8 @@ and you cannot become invisible.
 %%%%
 explore regen mutation
 
-As you explore the dungeon, your wounds will heal and your magic will replenish.
+You are invigorated by the discovery of the unknown. As you explore the dungeon,
+your wounds will heal and your magic will replenish.
 %%%%
 faith mutation
 

crawl-ref/source/dat/descript/species.txt

@@ -64,7 +64,8 @@ they may worship the Orc god Beogh.
 %%%%
 Human
 
-Humans advance quickly in levels and have equal abilities in most skills.
+Humans are natural explorers. Discovering new places refreshes and invigorates them.
+They are otherwise completely average.
 %%%%
 Tengu
 

crawl-ref/source/dat/species/human.yaml

@@ -4,7 +4,7 @@ name: Human
 difficulty: Intermediate
 difficulty_priority: 100
 aptitudes:
-  xp: 1
+  xp: 0
   hp: 0
   mp_mod: 0
   mr: 3
@@ -22,3 +22,6 @@ recommended_jobs:
   - JOB_CONJURER
   - JOB_NECROMANCER
   - JOB_ICE_ELEMENTALIST
+mutations:
+  1:
+    MUT_EXPLORE_REGEN: 1