Add zstd decompression support #611 (#648)

Closes #611.

emscripten/zstandard/compile.sh

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+emcc --memory-init-file 0 -O3 --closure 1 -s ENVIRONMENT="web" -s WASM=0 -s MALLOC="emmalloc" -s TOTAL_MEMORY=140247040 -s FILESYSTEM=0 -s DOUBLE_MODE=0 -s DYNAMIC_EXECUTION=0 -s MIN_IE_VERSION=11 -s EXPORT_NAME="ZD" -s MODULARIZE=1 -s EXPORTED_FUNCTIONS="['_malloc', '_free', '_ZSTD_createDStream', '_ZSTD_initDStream', '_ZSTD_decompressStream', '_ZSTD_isError', '_ZSTD_getErrorName', '_ZSTD_freeDStream', '_ZSTD_DStreamInSize', '_ZSTD_DStreamOutSize']" -s EXPORTED_RUNTIME_METHODS="['cwrap']" *.c -o zstddec.js

emscripten/zstandard/readme.md

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+# Source code and compilations of zstandard decompression library
+
+The file `zstddeclib.c` is the single-file source code for the zstandard decompressor (compressor is not included).
+It has been combined by following the instructions at https://github.com/facebook/zstd/tree/dev/contrib/single_file_libs. It can be used as the source file of an Emscripten webassembly or asm compilation.
+
+The source code can be compiled with Emscripten by setting up the Emscripten SDK and running the Linux script `./compile.sh` in this directory. This will produce `zstddec.js`. Further optimization and minification of this file may be possible by adjusting the commandline options in `compile.sh`.
+
+For a fuller Emscripten distribution of the full zstandard library for `node.js`, see https://github.com/yoshihitoh/zstd-codec/tree/develop/js/example.