Instead of checking the header in ZlibDecompressor::create(), we now
check it in read_some() when it is called for the first time. This
resolves a FIXME in the new DecompressionStream implementation.
These compressors will be used by w3c's CompressionStream, which can run
arbitrary JS, and thus never reach their "finish" steps. Let's not crash
the WebContent process if that happens.
Now we have an actual stream implementation that can read arbitrary
(dynamic codes aren't supported yet) deflate encoded data. Even if
the blocks are really large.
And all of that happens with a single buffer of 32KiB. DEFLATE is
amazing!
Previously, the implementation would produce one Vector<u8> which
would contain the whole decompressed data. That can be a lot and
even exhaust memory.
With these changes it is still necessary to store the whole input data
in one piece (I am working on this next,) but the output can be read
block by block. (That's not optimal either because blocks can be
arbitrarily large, but it's good for now.)
