Say "easily" instead of "sanely"

I can't correct it every time, but every time someone uses "insane"/
"sane" as synonyms for "bad"/"good", they're saying that being like
me is the same as being everything that's bad and wrong in the world...
so I'm changing this to a word that means approximately the same thing
but doesn't devalue any group of people.

src/libsyntax/ast.rs

@@ -313,7 +313,7 @@ enum expr_ {
     expr_fn(proto, fn_decl, blk, capture_clause),
     expr_fn_block(fn_decl, blk, capture_clause),
     // Inner expr is always an expr_fn_block. We need the wrapping node to
-    // sanely type this (a function returning nil on the inside but bool on
+    // easily type this (a function returning nil on the inside but bool on
     // the outside).
     expr_loop_body(@expr),
     expr_block(blk),