v0.22.0

Fixed

• A field named with a Rust keyword reached the wire with its r# prefix.
  #[derive(VgiArrow)] took each column name from Ident::to_string(), which
  keeps the raw-identifier marker: a field that has to be spelled r#type
  produced an Arrow column literally named "r#type". The VGI protocol has
  such a column — catalog_schema_contents_functions and _macros carry a
  type selector — so those methods could not be called by a Python, C++ or Go
  peer, which all send type. The prefix is now stripped on both the schema and
  the array-building path, matching how serde and the rest of the ecosystem
  treat raw identifiers.

• Map columns used arrow-rs's child names rather than the protocol's. The
  Vec<(String, V)> implementation built its entries struct as
  keys/values — arrow-rs's MapBuilder default — where pyarrow, the
  canonical Python protocol, the C++ extension and the Go worker all use
  key/value. Any map-valued field (tags, options, estimated_object_count)
  therefore went out with non-canonical child names. The read path was already
  positional, so only what is written changes.

Wire compatibility

Both fixes change bytes on the wire, which is why this is a minor bump rather
than a patch. Both move Rust toward the canonical Python protocol, so a Rust
worker talking to a Python, C++ or Go peer is strictly more correct after
upgrading — one of the two methods above did not work at all before. A
Rust-to-Rust pair is unaffected in the map case, since that decoder reads
entries positionally.