internals: Support characters beyond the first unicode plane (WIP)

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
 * on-screen keyboard
 * font manipulation, caching, rendering, and generation
 * VFAT code parses and generates utf16 dirents
 * WIN32 simulator reads and writes utf16 filenames

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

Known bugs:

  * Native players in 32-bit unicode mode generate mangled filename
    entries if they include UTF16 surrogate codepoints.  Root cause
    is unclear, and may reside in core dircache code.

Needs testing on:

 * windows simulator (16bit+32bit)

Change-Id: I193a00fe2a11a4181ddc82df2d71be52bf00b6e6

firmware/export/config.h

@@ -1461,4 +1461,11 @@ Lyre prototype 1 */
 #error "HAVE_LCD_SLEEP_SETTING requires HAVE_LCD_SLEEP"
 #endif
 
+// XXX Figure out a better place to put this?
+#ifdef UNICODE32
+#define ucschar_t unsigned int
+#else
+#define ucschar_t unsigned short
+#endif
+
 #endif /* __CONFIG_H__ */