Temporarily pin proc-macro2 to fix broken nightly builds

<dtolnay/proc-macro2#383> removed some
functionality we use due to a pending change to the upstream API.
Until the upstream API change is complete and the functionality is
restored in proc-macro2, pin an older version that still works
right now.

binrw_derive/Cargo.toml

@@ -16,7 +16,7 @@ proc-macro = true
 [dependencies]
 either = "1.8"
 owo-colors = { version = "3", optional = true }
-proc-macro2 = { version = "1", features = ["span-locations"] }
+proc-macro2 = { version = "=1.0.56", features = ["span-locations"] }
 quote = "1"
 syn = { version = "1", features = ["extra-traits", "fold", "full", "visit"] }