Formatting fixes in rust-exp.y

Formatting fixes in rust-exp.y

I noticed a few formatting buglets in rust-exp.y: A couple of lines
were too long, and a couple of parser rules did not follow the same
formatting as the rest of the code.

I'm checking this in as obvious.  Tested by rebuilding.

2018-03-06  Tom Tromey  <tom@tromey.com>

	* rust-exp.y: Formatting fixes.