Deprecate the nu_scripts version of stdlib-candidate (#1042)

Deprecates the existing `stdlib-candidate` directories in `nu_scripts`
since `std-rfc` has now transitioned to the main repo. Updates readme
and renamed directories.

stdlib-candidate-archive/std-rfc/aggregate/mod.nu

@@ -0,0 +1,135 @@
+def aggregate-default-ops [] {
+    {
+        min: {math min},
+        avg: {math avg},
+        max: {math max},
+        sum: {math sum},
+    }
+}
+
+def aggregate-col-name [col: cell-path, op_name: string]: [nothing -> string] {
+    $col | split cell-path | get value | str join "." | $"($in)_($op_name)"
+}
+
+def get-item-with-error [
+    col: cell-path,
+    opts: record<span: record<start: int, end: int>, items: bool>
+]: [table -> any] {
+    try {
+        get $col
+    } catch {
+        let full_cellpath = if $opts.items {
+            $col
+            | split cell-path
+            | prepend {value: items, optional: false}
+            | into cell-path
+        } else {
+            $col
+        }
+        error make {
+            msg: $"Cannot find column '($full_cellpath)'",
+            label: {
+                text: "value originates here",
+                span: $opts.span
+            },
+        }
+    }
+}
+
+def "error not-a-table" [span: record<start: int, end:int>] {
+    error make {
+        msg: "input must be a table",
+        label: {
+            text: "from here",
+            span: $span
+        },
+        help: "Are you using `group-by`? Make sure to use its `--to-table` flag."
+    }
+}
+
+# Run aggregate operations on output of `group-by --to-table`.
+#
+# # Example
+#
+# - group files by type and extension, and get stats about their sizes
+# ```nushell
+# >_ ls | group-by type { get name | path parse | get extension } --to-table | aggregate size
+# ```
+# 
+# - group data by multiple columns, and run custom aggregate operations
+# ```nushell
+# >_ open movies.csv
+#    | group-by Lead_Studio Genre --to-table
+#    | aggregate Worldwide_Gross Profitability --ops {avg: {math avg}, std: {math stddev}}
+# ```
+#
+# - run aggregate operations without grouping the input
+# ```nushell
+# >_ open movies.csv | aggregate Year
+# ```
+export def main [
+    --ops: record, # default = {min: {math min}, avg: {math avg}, max: {math max}, sum: {math sum}}  
+    ...columns: cell-path, # columns to perform aggregations on
+]: [
+    table -> table<count: int>,
+    record -> error,
+] {
+    let IN = $in
+    let md = metadata $in
+
+    let first = try { $IN | first } catch { error not-a-table $md.span }
+    if not (($first | describe) starts-with record) {
+        error not-a-table $md.span
+    }
+
+    let grouped = "items" in $first
+
+    let IN = if $grouped {
+        $IN
+    } else {
+        [{items: $IN}]
+    }
+
+    let agg_ops = $ops | default (aggregate-default-ops)
+
+    let results = $IN
+    | update items {|group|
+        let column_results = $columns
+        | each {|col| # col: cell-path
+            let column = $group.items | get-item-with-error $col {span: $md.span, items: $grouped}
+            let agg_results = $agg_ops | items {|op_name, op| # op_name: string, op: closure
+                $column | do $op | wrap (aggregate-col-name $col $op_name)
+            }
+
+            for r in $agg_results {
+                if ($r | describe) == error {
+                    return $r
+                }
+            }
+
+            $agg_results
+            | reduce {|it| merge $it}
+        }
+
+        # Manually propagate errors
+        for r in $column_results {
+            if ($r | describe) == error {
+                return $r
+            }
+        }
+
+        $column_results
+        | reduce --fold {} {|it| merge $it}
+        | insert count ($group.items | length)
+        | roll right  # put count as the first column
+    }
+
+    # Manually propagate errors
+    for r in $results {
+        if ($r.items | describe) == error {
+            return $r.items
+        }
+    }
+
+    $results | flatten items
+}