vcdecoder: add memory-based output interface

[zonque]

Currently, the VCDiffStreamingDecoder class expects applications to call into ::StartDecoding() with a dictionary, and then maintains an internal buffer (decoded_target_) to accumulate pending output bytes. These are then written through ::AppendNewOutputText() and ::FlushDecodedTarget().

The problem with this implementation is that the allocation of this intermediate buffer may fail, which will then result in the application abort with a trap.

This patch adds an alternative interface that does not lead to internal allocation. StartDecoding() may now optionally be called with an output buffer and a maximum length, and DecodeChunk() may be called without an OutputString parameter. If operated this way, the decoder will directly write to the provided memory buffer, which can, for instance, be mmap()ed on a block device.

The old interface is kept around for full compatibility.

This should address #12.

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[zonque]

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