Change tabstop from 4 to 8

This won't affect 4-space indentation in any way.

While the chance we find tab characters inside Rust sources is slim, it
is best to have the tab character displayed in accordance to terminals
and printers, where tabstop=8.

Even Vim doc says:

    Note: Setting 'tabstop' to any other value than 8 can make your file
    appear wrong in many places (e.g., when printing it).

Here's the full spec of settings for 4-space indentation in Vim:

set tabstop=8                   "A tab is 8 spaces
set expandtab                   "Always uses spaces instead of tabs
set softtabstop=4               "Insert 4 spaces when tab is pressed
set shiftwidth=4                "An indent is 4 spaces

More details:

    https://www.reddit.com/r/vim/wiki/tabstop

ftplugin/rust.vim

@@ -41,7 +41,7 @@ setlocal smartindent nocindent
 
 if get(g:, 'rust_recommended_style', 1)
 	let b:rust_set_style = 1
-	setlocal tabstop=4 shiftwidth=4 softtabstop=4 expandtab
+	setlocal tabstop=8 shiftwidth=4 softtabstop=4 expandtab
 	setlocal textwidth=99
 endif