Add version info for software library and on-disk structures

An annoying part of filesystems is that the software library can change
independently of the on-disk structures. For this reason versioning is
very important, and must be handled separately for the software and
on-disk parts.

In this patch, littlefs provides two version numbers at compile time,
with major and minor parts, in the form of 6 macros.

LFS_VERSION        // Library version, uint32_t encoded
LFS_VERSION_MAJOR  // Major - Backwards incompatible changes
LFS_VERSION_MINOR  // Minor - Feature additions

LFS_DISK_VERSION        // On-disk version, uint32_t encoded
LFS_DISK_VERSION_MAJOR  // Major - Backwards incompatible changes
LFS_DISK_VERSION_MINOR  // Minor - Feature additions

Note that littlefs will error if it finds a major version number that
is different, or a minor version number that has regressed.

lfs.h

@@ -22,6 +22,23 @@
 #include <stdbool.h>
 
 
+/// Version info ///
+
+// Software library version
+// Major (top-nibble), incremented on backwards incompatible changes
+// Minor (bottom-nibble), incremented on feature additions
+#define LFS_VERSION 0x00010002
+#define LFS_VERSION_MAJOR (0xffff & (LFS_VERSION >> 16))
+#define LFS_VERSION_MINOR (0xffff & (LFS_VERSION >>  0))
+
+// Version of On-disk data structures
+// Major (top-nibble), incremented on backwards incompatible changes
+// Minor (bottom-nibble), incremented on feature additions
+#define LFS_DISK_VERSION 0x00010001
+#define LFS_DISK_VERSION_MAJOR (0xffff & (LFS_DISK_VERSION >> 16))
+#define LFS_DISK_VERSION_MINOR (0xffff & (LFS_DISK_VERSION >>  0))
+
+
 /// Definitions ///
 
 // Type definitions