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/value.hh

@@ -9,6 +9,7 @@
 #include "value/context.hh"
 #include "input-accessor.hh"
 #include "source-path.hh"
+#include "print-options.hh"
 
 #if HAVE_BOEHMGC
 #include <gc/gc_allocator.h>
@@ -70,7 +71,7 @@ struct Pos;
 class StorePath;
 class EvalState;
 class XMLWriter;
-
+class Printer;
 
 typedef int64_t NixInt;
 typedef double NixFloat;
@@ -82,6 +83,7 @@ typedef double NixFloat;
 class ExternalValueBase
 {
     friend std::ostream & operator << (std::ostream & str, const ExternalValueBase & v);
+    friend class Printer;
     protected:
     /**
      * Print out the value
@@ -139,11 +141,9 @@ private:
 
     friend std::string showType(const Value & v);
 
-    void print(const SymbolTable &symbols, std::ostream &str, std::set<const void *> *seen, int depth) const;
-
 public:
 
-    void print(const SymbolTable &symbols, std::ostream &str, bool showRepeated = false, int depth = INT_MAX) const;
+    void print(EvalState &state, std::ostream &str, PrintOptions options = PrintOptions {});
 
     // Functions needed to distinguish the type
     // These should be removed eventually, by putting the functionality that's
@@ -364,10 +364,15 @@ public:
     inline void mkPrimOpApp(Value * l, Value * r)
     {
         internalType = tPrimOpApp;
-        app.left = l;
-        app.right = r;
+        primOpApp.left = l;
+        primOpApp.right = r;
     }
 
+    /**
+     * For a `tPrimOpApp` value, get the original `PrimOp` value.
+     */
+    PrimOp * primOpAppPrimOp() const;
+
     inline void mkExternal(ExternalValueBase * e)
     {
         clearValue();