Make Result::{unwrap, unwrap_err} require Show

`foo.ok().unwrap()` and `foo.err().unwrap()` are the fallbacks for types
that aren't `Show`.

Closes #13379
auto merge of #13479 : sfackler/rust/result-unwrap, r=cmr

`foo.ok().unwrap()` and `foo.err().unwrap()` are the fallbacks for types
that aren't `Show`.

Closes #13379