Merge #598: CI: Pin syn to 2.0.32

3d5cf3b CI: Pin syn to 2.0.32 (Daniela Brozzoni)

Pull request description:

ACKs for top commit:
  apoelstra:
    ACK 3d5cf3b

Tree-SHA512: d61a303bef9d14f9856134929b96be633c35cc506f377a704f1dd69b35e9a351a8537c903e0a6cefa0f51dae9534db91a94a993422442395ed31af83897a0896

Author: apoelstra

contrib/test.sh

@@ -24,6 +24,7 @@ fi
 if cargo --version | grep "1\.48\.0"; then
     cargo update -p once_cell --precise 1.13.1
     cargo update -p quote --precise 1.0.28
+    cargo update -p syn --precise 2.0.32
     cargo update -p proc-macro2 --precise 1.0.63
     cargo update -p serde_json --precise 1.0.99
     cargo update -p serde --precise 1.0.152