printer: rejigger how we use serde_derive

The idea is that by bringing derives in via serde's optional feature, it
was inhibiting compilation speed[1]. We try to fix that by depending on
`serde_derive` as a distinct dependency.

It does seem to improve overall compilation time, but only by about 0.5
seconds. With that said, my machine has a lot of cores, so it's possible
this will help more on less powerful CPUs.

[1]: https://old.reddit.com/r/rust/comments/17rd8ww/faster_compilation_with_the_parallel_frontend_in/k8igjlg/

crates/printer/src/jsont.rs

@@ -8,10 +8,7 @@
 
 use std::{borrow::Cow, path::Path};
 
-use {
-    base64,
-    serde::{Serialize, Serializer},
-};
+use {base64, serde::Serializer, serde_derive::Serialize};
 
 use crate::stats::Stats;
 
@@ -132,6 +129,7 @@ where
     T: AsRef<[u8]>,
     S: Serializer,
 {
+    use serde::Serialize;
     Data::from_bytes(bytes.as_ref()).serialize(ser)
 }
 
@@ -140,5 +138,6 @@ where
     P: AsRef<Path>,
     S: Serializer,
 {
+    use serde::Serialize;
     path.as_ref().map(|p| Data::from_path(p.as_ref())).serialize(ser)
 }

crates/printer/src/stats.rs

@@ -10,7 +10,7 @@ use crate::util::NiceDuration;
 /// When statistics are reported by a printer, they correspond to all searches
 /// executed with that printer.
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
-#[cfg_attr(feature = "serde", derive(serde::Serialize))]
+#[cfg_attr(feature = "serde", derive(serde_derive::Serialize))]
 pub struct Stats {
     elapsed: NiceDuration,
     searches: u64,