ReaderImplBase::ReadSection(): Small simplification
PackageWriterImpl: Remove superfluous method
package kit: ReaderImplBase: Remove superfluous scratch buffer
packagefs: CachedDataReader: Don't implement ReadData()

It's already done the same way in the base class.
BDataIO: Add Flush()
Move package kit Zlib* classes to support kit

Also move to B* namespace and no longer expose the zlib dependency in
the headers.
Add B[Zlib]CompressionAlgorithm

* BCompressionAlgorithm is a base class for classes that provide
  compression/decompression functionality. There are methods for
  compressing/decompressing a single buffer and factory methods for
  a compressing/decompressing input/output BDataIO.
* BZlibCompressionAlgorithm is a BCompressionAlgorithm implementation
  using zlib.
Test program for the B[Zlib]CompressionAlgorithm functionality
Switch package kit to BZlibCompressionAlgorithm

... and remove the Zlib{Compressor,Decompressor} API.