btrfs-tree: introduce lightweight classes for btrfs tree search operations

btrfs-tree provides classes for low-level access to btrfs tree objects.

An item class is provided to decode polymorphic btrfs item fields.

Several tree classes provide forward and backward iteration over raw
object items at different tree levels.

A csum tree class provides convenient access to csums by bytenr,
supporting all current btrfs csum types.

Wrapper classes for inode and subvol items provide direct access to
btrfs metadata fields without clumsy stat() wrappers or ioctls.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/Makefile

@@ -5,6 +5,7 @@ default: libcrucible.a
 
 CRUCIBLE_OBJS = \
 	bytevector.o \
+	btrfs-tree.o \
 	chatter.o \
 	city.o \
 	cleanup.o \