subscriber: don't enable tracing-core features by default (#2107)

This attempts to address #2106 by disabling the `tracing-core`
crate's default features in `tracing-subscriber`'s dependency.
Now, `tracing-core`'s optional "alloc" feature is only enabled
when "tracing-subscriber/alloc" is enabled.

Closes #2106

tracing-subscriber/Cargo.toml

@@ -38,7 +38,7 @@ valuable = ["tracing-core/valuable", "valuable_crate", "valuable-serde", "tracin
 local-time = ["time/local-offset"]
 
 [dependencies]
-tracing-core = { path = "../tracing-core", version = "0.1.22" }
+tracing-core = { path = "../tracing-core", version = "0.1.22", default-features = false }
 
 # only required by the filter feature
 tracing = { optional = true, path = "../tracing", version = "0.1", default-features = false }