RFC -> MCP

'Request for Comment' is indicative of a larger process than these are,
and can easily be confused with the larger Rust RFCs.
'Major Change Proposal' more accurately describes the process and also
has precedent in the language and compiler teams.

Author: jyn514

rfcs/migrate-to-async-hyper.md renamed to mcps/migrate-to-async-hyper.md

@@ -1,6 +1,6 @@
 # Summary
 
-This RFC proposes moving away from the current model of fetching new releases
+This MCP proposes moving away from the current model of fetching new releases
 to build, moving from using a timer to receiving webhooks.
 
 # Motivation