Add a Vim filetype plugin with useful tweaks.

This improves things like doc comment handling when you press Enter and
making using `gf` or `<C-W>f` work on a `use x;` statement in the
current directory.

src/etc/vim/ftplugin/rust.vim

+" Vim syntax file
+" Language:     Rust
+" Maintainer:   Chris Morgan <me@chrismorgan.info>
+" Last Change:  2013 Jul 6
+
+if exists("b:did_ftplugin")
+	finish
+endif
+let b:did_ftplugin = 1
+
+setlocal comments=s1:/*,mb:*,ex:*/,:///,://!,://
+setlocal commentstring=//%s
+setlocal formatoptions-=t formatoptions+=croqnlj
+
+" This includeexpr isn't perfect, but it's a good start
+setlocal includeexpr=substitute(v:fname,'::','/','g')
+
+" NOT adding .rc as it's being phased out (0.7)
+setlocal suffixesadd=.rs
+
+if exists("g:ftplugin_rust_source_path")
+    let &l:path=g:ftplugin_rust_source_path . ',' . &l:path
+endif
+
+let b:undo_ftplugin = "setlocal formatoptions< comments< commentstring< includeexpr< suffixesadd<"