Update to proc-macro2 1.0.57 to unblock proc_macro_span changes

Author: dtolnay

Cargo.lock

@@ -2638,9 +2638,9 @@ checksum = "dbf0c48bc1d91375ae5c3cd81e3722dff1abcf81a30960240640d223f59fe0e5"
 
 [[package]]
 name = "proc-macro2"
-version = "1.0.56"
+version = "1.0.57"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "2b63bdb0cd06f1f4dedf69b254734f9b45af66e4a031e42a7480257d9898b435"
+checksum = "c4ec6d5fe0b140acb27c9a0444118cf55bfbb4e0b259739429abb4521dd67c16"
 dependencies = [
  "unicode-ident",
 ]