Add more tests

[chmp]

• Test more combinations of nested arrays. E.g., currently not tested:
  • Struct with FixedSizeList child
  • Struct with Union child
  • Struct with Map child
  • Struct with Dictionary child
  • Struct with FixedSizeBinary child
• Add more integration tests (write arrays in Python reconstruct in Rust, and vice versa)
• Test deserialization on slices (i.e., with nonzero offset)