Update and rename 00X3-stride-semantics.md to 0051-stride-semantics.md

Author: Dave Abrahams

proposals/00X3-stride-semantics.md renamed to proposals/0051-stride-semantics.md

@@ -1,6 +1,6 @@
 # Conventionalizing `stride` semantics
 
-* Proposal: SE-00NN
+* Proposal: SE-0051
 * Author(s): [Erica Sadun](http://github.com/erica)
 * Status: TBD
 * Review manager: TBD
@@ -145,4 +145,4 @@ Renaming two stride functions and adding a third does not change or break existi
 
 ## Alternatives Considered
 
-The only alternative at this time is "no change" to existing semantics.
+The only alternative at this time is "no change" to existing semantics.