Kill redundant imports.

Recently Rust starting warning about redundant imports, e.g.:
```
warning: the item `Extend` is imported redundantly
   --> runtime/src/lib.rs:39:5
    |
39  | use std::iter::Extend;
    |     ^^^^^^^^^^^^^^^^^
```

This change removes all such imports.

plugin/src/arch/aarch64/ast.rs

@@ -1,4 +1,3 @@
-use syn;
 use proc_macro2::Span;
 
 use crate::common::{Size, Jump};

plugin/src/arch/x64/ast.rs

@@ -1,4 +1,3 @@
-use syn;
 use proc_macro2::Span;
 
 use crate::common::{Size, Jump};

plugin/src/serialize.rs

@@ -1,4 +1,3 @@
-use syn;
 use syn::parse;
 use syn::spanned::Spanned;
 use proc_macro2::{Span, TokenStream, TokenTree, Literal};

runtime/src/components.rs

@@ -616,8 +616,7 @@ impl LitPool {
 #[cfg(test)]
 mod tests {
     use crate::*;
-    use std::fmt::Debug;
-    use relocations::{Relocation, RelocationSize};
+    use relocations::RelocationSize;
 
     #[test]
     fn test_litpool_size() {

runtime/src/lib.rs

@@ -36,7 +36,6 @@ use crate::components::{MemoryManager, LabelRegistry, RelocRegistry, ManagedRelo
 use crate::relocations::Relocation;
 
 use std::hash::Hash;
-use std::iter::Extend;
 use std::sync::{Arc, RwLock, RwLockReadGuard};
 use std::io;
 use std::error;