Bound the length of nls completions (#2073)

* Limit the sizes of contracts in completion details

* Improve the nls benches a little

* Use guards

* Fix bench build

* Generics cleanups, scope cleanups

core/benches/arrays.rs

@@ -3,11 +3,13 @@
 use criterion::criterion_main;
 use nickel_lang_core::term::{array::ArrayAttrs, Number, RichTerm, Term};
 use nickel_lang_utils::{bench::criterion_config, bench::EvalMode, ncl_bench_group};
-use pretty::{BoxAllocator, DocBuilder, Pretty};
+use pretty::{DocBuilder, Pretty};
 
 /// Generates a pseaudo-random Nickel array as a string.
 #[cfg(not(feature = "benchmark-ci"))]
 fn ncl_random_array(len: usize) -> String {
+    use nickel_lang_core::pretty::Allocator;
+
     let m = 2_u64.pow(32);
     let a = 1664525;
     let c = 1013904223;
@@ -24,7 +26,8 @@ fn ncl_random_array(len: usize) -> String {
         numbers.into_iter().collect(),
         ArrayAttrs::default(),
     ));
-    let doc: DocBuilder<_, ()> = xs.pretty(&BoxAllocator);
+    let alloc = Allocator::default();
+    let doc: DocBuilder<_, ()> = xs.pretty(&alloc);
     let mut out = Vec::new();
     doc.render(80, &mut out).unwrap();
     String::from_utf8(out).unwrap()