Add optional Zstandard compression for state

This can be particularly useful when using the persistent fields feature
to store JSON or other large textual documents.

.github/workflows/docs.yml

@@ -51,7 +51,7 @@ jobs:
           # conflicting iced features. We also don't want to use `--workspace`
           # here because that would also document our plugins and binary crates.
           args: >-
-            --features docs,simd,standalone --no-deps
+            --features docs,simd,standalone,zstd --no-deps
             -p nih_plug
             -p nih_plug_derive
             -p nih_plug_egui

.github/workflows/test.yml

@@ -60,7 +60,7 @@ jobs:
           command: build
           # Don't use --all-features as that will enable a whole bunch of
           # conflicting iced features
-          args: --workspace --features "simd,standalone"
+          args: --workspace --features "simd,standalone,zstd"
       - name: Run the tests
         uses: actions-rs/cargo@v1
         with: