packagefs: Use object caches for the most used node classes.

Slab statistics from KDL show that on a stock nightly image
(i.e. no additional packages besides the standards installed)
there are 43848 *objects* (not bytes) in the PackageNodeAttribute
cache, and 25090 in the PackageFile cache, so this seems more than
worth it.

The last commit seems to reduce memory usage at boot by about 1%,
this commit seems to not affect it at all; but it is a significant
performance optimization and on systems with more packages installed
the effect may be very noticeable.

Change-Id: I676a642ed6003f82b14396e1f02684575d899362

packagefs: Add support for hpkg format V1

* Add union-like class PackageData which wraps the V1 and V2
BPackageData classes.
* GlobalFactory: Create a data reader depending on the data format
version.
* Package: Add a loader for V1 format and try that, if the other one
fails.

packagefs: Share string instances globally

Introduce a class String which refers to shared private data that is
registered in a global hash table (in class StringPool) and use the
class consequently. This eliminates duplicate allocations for the same
string and also speeds up tests for equality. There's quite a bit
overhead for the management structures (apparently even more than for
the string data itself), but due to almost all strings being used
multiple times this still almost halves the memory usage for string
data.

packagefs: Organize sources into subdirectories

packagefs: Add support for hpkg format V1

* Add union-like class PackageData which wraps the V1 and V2
BPackageData classes.
* GlobalFactory: Create a data reader depending on the data format
version.
* Package: Add a loader for V1 format and try that, if the other one
fails.

packagefs: Share string instances globally

Introduce a class String which refers to shared private data that is
registered in a global hash table (in class StringPool) and use the
class consequently. This eliminates duplicate allocations for the same
string and also speeds up tests for equality. There's quite a bit
overhead for the management structures (apparently even more than for
the string data itself), but due to almost all strings being used
multiple times this still almost halves the memory usage for string
data.

packagefs: Organize sources into subdirectories