Provide assistance for writing an impl block

When I am at a type, I can select a "generate impl" assist and it will generate a block right below the type. That's useful, but often I am somewhere else in the project (might not even be the same file), and I want to generate an impl block here. Would be great if there was some way to get the assist there as well. :)

What I usually try is to write impl TypeName, and then my hope would be that (a) I get auto-complete (including auto-import) for the type name, and (b) I can then select an assist to generate an impl block, and it will turn this into

impl<'a, T: Bound> TypeName<'a, T> {
  /* cursor here */
}

Start by locating the existing "generate impl" assist that works when the cursor is at a type, then inspect how it determines the target type and creates the impl block. The work is done when the assist is available after writing impl TypeName elsewhere and produces the requested generic and lifetime-aware block, including the cursor position.