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/target/hosted/filesystem-win32.h

@@ -27,10 +27,10 @@
 /* filesystem-win32.c contains some string functions that could be useful
  * elsewhere; just move them away to unicode.c or something if they prove
  * so. */
-size_t strlcpy_ucs2utf8(char *buffer, const unsigned short *ucs,
-                        size_t bufsize);
+size_t strlcpy_utf16utf8(char *buffer, const unsigned short *utf16,
+                         size_t bufsize);
 
-#define strlcpy_from_os strlcpy_ucs2utf8
+#define strlcpy_from_os strlcpy_utf16utf8
 #endif /* __MINGW32__ */
 
 #endif /* !OSFUNCTIONS_DECLARED */