compression: fix decomp buffer mgt

[fia0]

Due to the usage of AlignedStorage and its Wrapper^n Buf when storing and
serializing buffers length information is lost and the total length will simply
be set to the capacity. This is wrong. While serialization libraries seem to
handle this well enough, the decompression structure used from zstd did not
(This could be a bug though). This patch reworks the de/compression part to do 5
things:

• store the length of the original data alongside the compressed to ensure the
  minimal amount of reallocation on decompression (1); before for each block a
  reallocation happened bc of how the zstd decompression writer works
  internally

• minimize the amount of copies during compression and decompression; the
  zstd writer holds an internal buffer which, to my understanding, always
  buffers the intermediate results and then copies it to the final buffer. We
  avoid this structure now entirely to reduce the amount of copies

• estimate the size of the compressed results inbefore to reduce realloc's

• rework the compression interface to be similar to a one-and-done process,
  compressors and decompressors might be reused later on (needs an additional
  reinit), but the append-append-compress workflow was never used and comes with
  inefficienct reallocs

• remove redundant memcpy when using the None compression algorithm

[fia0]

This PR also adds some basic tests to the zstd module in compression.

[fia0]

Tests still seem to work fine, will merge now.