Track nondeterministic uniffi-dart generated binding order

[reez]

After updating to uniffi-dart v0.2.0+v0.31.1 in #67, repeated runs of ./scripts/generate_bindings.sh can rewrite lib/bdk.dart with FFI converter declaration-order churn even when the generated class set is unchanged.

[reez]

the churn appears not from a bdk-dart API/runtime change.

repeated generation keeps the same generated converter class set, but reorders the FfiConverter* helper declarations in lib/bdk.dart.

[reez]

opened issue: Uniffi-Dart/uniffi-dart#133

opened pr: Uniffi-Dart/uniffi-dart#134