subscriber: fix imports with std only

Turns out that if `std` is enabled but `smallvec` is not, we have some
missing imports. This fixes the build when a crate passes
`default-features = "false` but then enables `std`.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

tracing-subscriber/src/filter/directive.rs

@@ -1,9 +1,9 @@
 use crate::filter::level::{self, LevelFilter};
+#[cfg(not(feature = "smallvec"))]
+use alloc::vec;
 #[cfg(not(feature = "std"))]
-use alloc::{
-    string::String,
-    vec::{self, Vec},
-};
+use alloc::{string::String, vec::Vec};
+
 use core::{cmp::Ordering, fmt, iter::FromIterator, slice, str::FromStr};
 use tracing_core::Metadata;
 /// Indicates that a string could not be parsed as a filtering directive.

tracing-subscriber/src/registry/mod.rs

@@ -234,8 +234,9 @@ pub struct Scope<'a, R> {
 feature! {
     #![any(feature = "alloc", feature = "std")]
 
-    #[cfg(not(feature = "std"))]
+    #[cfg(not(feature = "smallvec"))]
     use alloc::vec::{self, Vec};
+
     use core::{fmt,iter};
 
     /// An iterator over the parents of a span, ordered from root to leaf.