Change traversal limit on machines with short pointers to 8192 words

This is 65536 bytes, which will exactly fill the memory addressable by
16-bit pointers. It is almost certain that legitimate messages will
run out of buffer space before hitting this limit.

Author: saulrh

capnp/src/message.rs

@@ -120,7 +120,7 @@ pub const DEFAULT_READER_OPTIONS: ReaderOptions = ReaderOptions {
 
 #[cfg(target_pointer_width = "16")]
 pub const DEFAULT_READER_OPTIONS: ReaderOptions = ReaderOptions {
-    traversal_limit_in_words: Some(1024),
+    traversal_limit_in_words: Some(8 * 1024),
     nesting_limit: 64,
 };