`dev_urandom_fallback` presently does nothing, so don't bother adding
it as a matrix dimension.
Because `dev_urandom_fallback` does nothing,
`--no-default-features --features=alloc` is effectively the same as the
default feature set, so don't add it as a matrix dimension.
`--all-features` is no longer appropriate after the ARM hardware feature
disabling features were added, so replace it with what we really want,
`--features=std,slow_tests`.
In the coverage section, `--features=alloc,dev_urandom_fallback,std` is
the same as `--features=std` since `alloc and `urandom_fallback` are
default features.
8f042cef71