ct/lc_ct_cmp: improve interface

Return 0xffffffff if arguments compare equal, 0 otherwise. Change all
consumers accordingly.

README

@@ -15,9 +15,7 @@ Utilities
 ---------
 
 - Constant-time operations
-  - [/] compare: returns `0` if match, non-`0` otherwise. The non-`0`
-    case might leak information. Would be better to return `0xffffffff`
-    if match, `0` otherwise.
+  - [x] compare
 
 Hash
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