clippy.toml: bump up large error threshold

On 12.x apparently the "large error" (actually a copy of Self) returned
by the plan API is a fair bit larger than it is on master, so we need
to increase this threshold by a bunch.

Author: apoelstra

clippy.toml

@@ -1,3 +1,3 @@
 msrv = "1.56.1"
 # plan API returns Self as an error type for an large-ish enum
-large-error-threshold = 256
+large-error-threshold = 512