rust-mode: Default rust-indent-offset to 4, not default-tab-width

rust-mode: Default rust-indent-offset to 4, not default-tab-width

default-tab-width is standardly 8, but most programmers and style
guides prefer an indentation width smaller than that.  Rust itself
uses 4 space indents.  Most other Emacs modes define the indentation
width as 4 or 2 spaces, independently of the width of a Tab character.
Depending on default-tab-width makes especially little sense for
rust-mode because it sets indent-tabs-mode to nil.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>