unicode: Support characters beyond the first unicode plane

We used 16-bit variables to store the 'character code' everywhere but
this won't let us represent anything beyond U+FFFF.

This patch changes those variables to a custom type that can be 32 or 16
bits depending on the build, and adjusts numerous internal APIs and
datastructures to match.  This includes:

 * utf8decode() and friends
 * font manipulation, caching, rendering, and generation
 * on-screen keyboard
 * FAT filesystem (parsing and generating utf16 LFNs)
 * WIN32 simulator platform code

Note that this patch doesn't _enable_ >16bit unicode support; a followup
patch will turn that on for appropriate targets.

Appears to work on:

  * hosted linux, native, linux simulator in both 16/32-bit modes.

Needs testing on:

  * windows and macos simulator (16bit+32bit)

Change-Id: Iba111b27d2433019b6bff937cf1ebd2c4353a0e8

firmware/include/lru.h

@@ -33,6 +33,7 @@ struct lru
     void *_base;
 };
 
+/* LRU_SLOT_OVERHEAD is the fixed portion of struct lru_node */
 #define LRU_SLOT_OVERHEAD (2 * sizeof(short))
 
 /* Create LRU list with specified size from buf. */
@@ -45,4 +46,3 @@ void *lru_data(struct lru* pl, short handle);
 void lru_traverse(struct lru* pl, void (*callback)(void* data));
 
 #endif /* LRU_H */
-