vim: Add a concealer for <->

This may break indentation though. However, without it, vim
sees "<->" and interprets it as a "<", folloed by "->", which
it conceals as a right arrow, so I'm not sure which is worse.

Author: erickt

src/etc/vim/after/syntax/rust.vim

@@ -15,6 +15,10 @@ syn match rustRightArrowHead contained ">" conceal cchar= 
 syn match rustRightArrowTail contained "-" conceal cchar=⟶
 syn match rustNiceOperator "->" contains=rustRightArrowHead,rustRightArrowTail
 
+syn match rustLeftRightArrowHead contained ">" conceal cchar= 
+syn match rustLeftRightArrowTail contained "<-" conceal cchar=⟷
+syn match rustNiceOperator "<->" contains=rustLeftRightArrowHead,rustLeftRightArrowTail
+
 syn match rustFatRightArrowHead contained ">" conceal cchar= 
 syn match rustFatRightArrowTail contained "=" conceal cchar=⟹
 syn match rustNiceOperator "=>" contains=rustFatRightArrowHead,rustFatRightArrowTail