Optimize Equal using %record/split_pair%

The std.contract.Equal contract needs to be lazy, which means that it
re-implement equality logic in pure Nickel code, which has proven to be
very slow (#1930).

This commit replaces the field difference operation, implemented using a
left fold, by the new builtin operator `%record/split_pair%`, which
shows a performance improvement of around 300% for the example of #1930.

core/src/eval/operation.rs

@@ -2576,7 +2576,7 @@ impl<R: ImportResolver, C: Cache> VirtualMachine<R, C> {
 
                 let Term::Record(record1) = t1 else {
                     return Err(mk_type_error!(
-                        "record/full_difference",
+                        "record/split_pair",
                         "Record",
                         1,
                         t1.into(),
@@ -2586,7 +2586,7 @@ impl<R: ImportResolver, C: Cache> VirtualMachine<R, C> {
 
                 let Term::Record(record2) = t2 else {
                     return Err(mk_type_error!(
-                        "record/full_difference",
+                        "record/split_pair",
                         "Record",
                         2,
                         t2.into(),

core/src/term/mod.rs

@@ -1848,7 +1848,7 @@ impl fmt::Display for BinaryOp {
             RecordFieldIsDefined(RecordOpKind::ConsiderAllFields) => {
                 write!(f, "record/field_is_defined_with_opts")
             }
-            Self::RecordSplitPair => write!(f, "record/full_difference"),
+            Self::RecordSplitPair => write!(f, "record/split_pair"),
             Self::RecordDisjointMerge => write!(f, "record/disjoint_merge"),
             ArrayConcat => write!(f, "array/concat"),
             ArrayAt => write!(f, "array/at"),

core/stdlib/std.ncl

@@ -841,45 +841,14 @@
         ```
       "%
       =
-        let fields_diff
-          | doc m%"
-            Compute the difference between the fields of two records.
-            `fields_diff` isn't concerned with the actual values themselves, but
-            just with field names.
-
-            Return a record of type
-            `{extra : Array String, missing: Array String}`, relative to the
-            first argument `constant`.
-          "%
-          = fun constant value =>
-            let diff =
-              value
-              |> std.record.fields
-              |> std.array.fold_left
-                (
-                  fun acc field =>
-                    if std.record.has_field field acc.rest then
-                      {
-                        extra = acc.extra,
-                        rest = std.record.remove field acc.rest,
-                      }
-                    else
-                      {
-                        extra = std.array.append field acc.extra,
-                        rest = acc.rest,
-                      }
-                )
-                { extra = [], rest = constant }
-            in
-            { extra = diff.extra, missing = std.record.fields diff.rest }
-          in
         let blame_fields_differ = fun qualifier fields ctr_label =>
           let plural = if %array/length% fields == 1 then "" else "s" in
           ctr_label
           |> label.with_message "%{qualifier} field%{plural} `%{std.string.join ", " fields}`"
           |> label.append_note "`std.contract.Equal some_record` requires that the checked value is equal to the record `some_record`, but the sets of their fields differ."
           |> blame
         in
+
         fun constant =>
           let constant_type = %typeof% constant in
           let check_typeof_eq = fun ctr_label value =>
@@ -916,11 +885,17 @@
               let contract_map = %record/map% constant (fun _key => Equal) in
               fun ctr_label value =>
                 let value = check_typeof_eq ctr_label value in
-                let diff = fields_diff constant value in
-                if %array/length% diff.extra != 0 then
-                  blame_fields_differ "extra" diff.extra ctr_label
-                else if %array/length% diff.missing != 0 then
-                  blame_fields_differ "missing" diff.missing ctr_label
+                let split_result = %record/split_pair% constant value in
+                if split_result.right_only != {} then
+                  blame_fields_differ
+                    "extra"
+                    (%record/fields% split_result.right_only)
+                    ctr_label
+                else if split_result.left_only != {} then
+                  blame_fields_differ
+                    "missing"
+                    (%record/fields% split_result.left_only)
+                    ctr_label
                 else
                   %contract/record_lazy_apply%
                     ctr_label