Support `beginning-of-line-text` and `(fill-paragraph 't)` in comments

I'm not sure if this should be two issues or one, but they feel related.

When programming in C/C++ with cc-mode, when the cursor is inside a comment and I hit M-<home> (which is bound to beginning-of-line-text), then the cursor moves to the first character of the comment text after the comments symbols, e.g.:

   // Comment text
      ^ `beginning-of-line-text` moves the cursor here

In rust-mode, the cursor moves to the first non-whitespace character (same as back-to-indentation).

Possibly related: when I hit C-u M-q (bound to (fill-paragraph 't)) to justify a paragraph in a comment, rust-mode adds extra spaces to the begnining of some lines:

   /// Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do
   ///  in reprehenderit  in  voluptate velit  esse  cillum dolore  eu
   /// fugiat nulla pariatur.

whereas cc-mode is able to justify the text correctly:

   /// Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed do
   /// in  reprehenderit  in voluptate  velit  esse  cillum dolore  eu
   /// fugiat nulla pariatur.

I believe both issues may be related to identifying and setting the fill-prefix when these functions are called inside comments. It would be nice to have those working as in cc-mode.

Start by examining rust-mode's handling of comments when beginning-of-line-text and (fill-paragraph 't) run, focusing on how fill-prefix is identified. Compare the behavior with cc-mode using the examples in the issue; done means comment text starts after its markers and paragraph filling avoids extra leading spaces.