First implementation of a VLArray store

[FrancescAlted]

VLArray is a wrapper around one SChunk, with one logical entry per physical chunk.

Entries are:

• serialized with msgpack
• compressed before being stored
• decompressed and deserialized on retrieval

Supported value types are:

• bytes
• str
• int
• float
• bool
• None
• list
• tuple
• dict

The implementation also required a prerequisite fix in the sibling c-blosc2 repository so a single SChunk can hold variable-sized chunks across multiple chunks, not just a smaller last chunk.