Make Chunk compression field a char[4]

[jhurliman]

Right now, the compression field in Chunk is a variable-length string. This places the actual chunk payload at a variable offset, and requires parsing the compression string to determine the chunk payload length. If compression was instead a fixed-length char[4], we would know the chunk payload size immediately after parsing the record length and it would avoid an additional allocation for the std::string compression.

uncompressed would be [0x00, 0x00, 0x00, 0x00] or little-endian uint32_t 0
lz4 would be [0x6C, 0x7A, 0x34, 0x00] or little-endian uint32_t 3439212
zstd would be [0x7A, 0x73, 0x74, 0x64] or little-endian uint32_t 1685353338

[defunctzombie]

After discussion we've decided to stick with string. We were not swayed enough yet that this is annoying enough to require a new type to reason about.