varlink / rust / 31330154543

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fix: generator fixes for keyword identifiers, null optionals and Default (#152)

Three `varlink_generator` fixes/improvements, found while generating
client code for systemd's `io.systemd.*` interfaces (which use `type`
and friends as field names liberally). They are bundled because they
touch the same emission sites and share one regenerated golden file —
happy to split if preferred.

## 1. Keywords that cannot be raw identifiers

The generator blindly prefixes every identifier with `r#`, but `self`,
`Self`, `crate` and `super` cannot be raw identifiers —
`syn::parse_str("r#self")` fails, so the generator panics on any
interface using them as a type, field, variant or parameter name.

New `safe_ident()` suffixes those four with `_` and pairs them with
`#[serde(rename = "...")]` so the wire name is unchanged. All other
names keep the existing `r#` treatment (no rename needed — serde already
strips the `r#` prefix).

## 2. Null optionals in custom types

Method argument and reply structs already annotate optional fields with
`#[serde(skip_serializing_if = "Option::is_none")]`, but typedef structs
don't. A minimally-populated custom type therefore serializes every
absent field as `null`, which servers may reject — systemd, for example,
treats an explicit `null` as an attempt to set a property rather than
omitting it. Typedef structs now get the same annotation.

## 3. `derive(Default)`

All generated structs and enums now derive `Default` (enums mark the
first variant `#[default]` — this only affects `Enum::default()`, never
the wire encoding). This makes sparsely-populated argument structs
ergonomic:

```rust
let ctx = UnitContext { description: Some("test".into()), ..Default::default() };
```

which matters for interfaces whose types carry dozens of optional
fields.

## Tests

`tests/org.example.complex.varlink` gains `ReservedEnum`, a `Reserved`
type and a `Baz` method exercising the non-raw-able keywords as type,
field, variant and method-parameter ... (continued)

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