10418: Add whitespace between lifetime and mut keyword in "expand macro" command r=lnicola a=nathanwhit Before, we were only adding whitespace between a lifetime and the following ident, which produced invalid code for mutable references. Before this PR: ```rust macro_rules! foo { () => { pub struct Foo<'a> { foo: &'a mut str, } }; } foo!(); // <- expand macro here ``` expanded to ```rust pub struct Foo< 'a>{ foo: & 'amut str, } ``` with this PR, it expands to ```rust pub struct Foo< 'a>{ foo: & 'a mut str, } ``` Co-authored-by: nathan.whitaker <[email protected]>