pretty-printer should parenthesize this expr_assert

[jruderman]

fn f() { (assert (10 == 10)) = 10 }

becomes

fn f() { assert (10 == 10) = 10 }

which becomes

fn f() { assert (10 == 10 = 10) }

[brson]

The original code is not valid rust since the result of assert is not an lvalue, and I don't know that it's reasonable or worth the effort to say the pretty-printer should do anything in particular for invalid code. I can not come up with a similar test case that type checks.

[catamorphism]

Closing for same reason as #928