Unify and refactor value printing

Previously, there were two mostly-identical value printers -- one in
`libexpr/eval.cc` (which didn't force values) and one in
`libcmd/repl.cc` (which did force values and also printed ANSI color
codes).

This PR unifies both of these printers into `print.cc` and provides a
`PrintOptions` struct for controlling the output, which allows for
toggling whether values are forced, whether repeated values are tracked,
and whether ANSI color codes are displayed.

Additionally, `PrintOptions` allows tuning the maximum number of
attributes, list items, and bytes in a string that will be displayed;
this makes it ideal for contexts where printing too much output (e.g.
all of Nixpkgs) is distracting. (As requested by @roberth in
https://github.com/NixOS/nix/pull/9554#issuecomment-1845095735)

Please read the tests for example output.

Future work:
- It would be nice to provide this function as a builtin, perhaps
  `builtins.toStringDebug` -- a printing function that never fails would
  be useful when debugging Nix code.
- It would be nice to support customizing `PrintOptions` members on the
  command line, e.g. `--option to-string-max-attrs 1000`.

src/libutil/english.cc

@@ -0,0 +1,18 @@
+#include "english.hh"
+
+namespace nix {
+
+std::ostream & pluralize(
+    std::ostream & output,
+    unsigned int count,
+    const std::string_view single,
+    const std::string_view plural)
+{
+    if (count == 1)
+        output << "1 " << single;
+    else
+        output << count << " " << plural;
+    return output;
+}
+
+}

src/libutil/english.hh

@@ -0,0 +1,18 @@
+#pragma once
+
+#include <iostream>
+
+namespace nix {
+
+/**
+ * Pluralize a given value.
+ *
+ * If `count == 1`, prints `1 {single}` to `output`, otherwise prints `{count} {plural}`.
+ */
+std::ostream & pluralize(
+    std::ostream & output,
+    unsigned int count,
+    const std::string_view single,
+    const std::string_view plural);
+
+}