Reduce code bloat from assert!()

Assertions without a message get a generated message that consists of a
prefix plus the stringified expression that is being asserted. That
prefix is currently a unique string, while a static string would be
sufficient and needs less code.
auto merge of #8150 : dotdash/rust/assert_bloat, r=huonw

Assertions without a message get a generated message that consists of a
prefix plus the stringified expression that is being asserted. That
prefix is currently a unique string, while a static string would be
sufficient and needs less code.