Adds utilities for "advanced" methods of compression with lz4
and zstd -- with streaming (a history buffer persisted across messages)
and/or precomputed dictionaries.
This patch is mostly just glue needed to use the underlying
libraries with discontiguous input and output buffers, and for reusing the
same compressor context objects across messages. It doesn't contain
any innovations of its own.
There is one "design decision" in the patch. The block format of LZ4
doesn't contain the length of the compressed blocks. At decompression
time, that length must be delivered to the decompressor by a channel
separate to the compressed block itself. In `lz4_cstream`, we deal
with that by prepending a variable-length integer containing the
compressed size to each compressed block. This is suboptimal for
single-fragment messages, since the user of lz4_cstream is likely
going to remember the length of the whole message anyway,
which makes the length prepended to the block redundant.
But a loss of 1 byte is probably acceptable for most uses.
