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/libexpr/print-options.hh

@@ -0,0 +1,52 @@
+#pragma once
+/**
+ * @file
+ * @brief Options for printing Nix values.
+ */
+
+#include <limits>
+
+namespace nix {
+
+/**
+ * Options for printing Nix values.
+ */
+struct PrintOptions
+{
+    /**
+     * If true, output ANSI color sequences.
+     */
+    bool ansiColors = false;
+    /**
+     * If true, force values.
+     */
+    bool force = false;
+    /**
+     * If true and `force` is set, print derivations as
+     * `«derivation /nix/store/...»` instead of as attribute sets.
+     */
+    bool derivationPaths = false;
+    /**
+     * If true, track which values have been printed and skip them on
+     * subsequent encounters. Useful for self-referential values.
+     */
+    bool trackRepeated = true;
+    /**
+     * Maximum depth to evaluate to.
+     */
+    size_t maxDepth = std::numeric_limits<size_t>::max();
+    /**
+     * Maximum number of attributes in an attribute set to print.
+     */
+    size_t maxAttrs = std::numeric_limits<size_t>::max();
+    /**
+     * Maximum number of list items to print.
+     */
+    size_t maxListItems = std::numeric_limits<size_t>::max();
+    /**
+     * Maximum string length to print.
+     */
+    size_t maxStringLength = std::numeric_limits<size_t>::max();
+};
+
+}