UInt24Array: Array of unsigned ints encoded in 3 bytes.

UInt24Array: Array of unsigned ints encoded in 3 bytes.

The object size index stores positions of objects in the main
index (when ordered by sha1). These positions are per-pack and usually
a pack has <16 million objects (there are exceptions but rather
rare). It could save some memory storing these positions in three bytes
instead of four. Note that these positions are sorted and always positive.

Implement a wrapper around a byte[] to access and search "ints" while
they are stored as unsigned 3 bytes.

Change-Id: Iaa26ce8e2272e706e35fe4cdb648fb6ca7591972