Auto merge of #53824 - ljedrz:begone_onevector, r=michaelwoerister

Remove OneVector, increase related SmallVec capacities

Removes the `OneVector` type alias (equivalent to `SmallVec<[T; 1]>`); it is used in scenarios where the capacity of 1 is often exceeded, which might be nullifying the performance wins (due to spilling to the heap) expected when using `SmallVec` instead of `Vec`.

The numbers I used in this PR are very rough estimates - it would probably be a good idea to adjust some/all of them, which is what this proposal is all about.

It might be a good idea to additionally create some local type aliases for the `SmallVec`s in the `Folder` trait, as they are repeated in quite a few spots; I'd be happy to apply this sort of adjustments.

src/test/run-pass-fulldeps/auxiliary/issue-16723.rs

@@ -17,9 +17,10 @@ extern crate syntax;
 extern crate rustc;
 extern crate rustc_data_structures;
 extern crate rustc_plugin;
+#[macro_use] extern crate smallvec;
 extern crate syntax_pos;
 
-use rustc_data_structures::small_vec::OneVector;
+use smallvec::SmallVec;
 use syntax::ext::base::{ExtCtxt, MacResult, MacEager};
 use syntax::tokenstream;
 use rustc_plugin::Registry;
@@ -31,8 +32,8 @@ pub fn plugin_registrar(reg: &mut Registry) {
 
 fn expand(cx: &mut ExtCtxt, _: syntax_pos::Span, _: &[tokenstream::TokenTree])
           -> Box<MacResult+'static> {
-    MacEager::items(OneVector::from_vec(vec![
+    MacEager::items(smallvec![
         quote_item!(cx, struct Struct1;).unwrap(),
         quote_item!(cx, struct Struct2;).unwrap()
-    ]))
+    ])
 }