Prefer std::string over std::wstring. (#179)

In the past, FTXUI switched from std::string to std::wstring to support
fullwidth characters. The reasons was that fullwidth characters can be
stored inside a single wchar_t.

Then FTXUI added support for combining characters. A single glygh
doesn't even fit a wchar_t. Instead, a glyph can be arbitrary large.

The usage of wstring doesn't really fit the new model and have several
drawbacks:
1. It doesn't simplify the implementation of FTXUI, because of combining
   characters.
2. It reduces drawing performance by 2x.
3. It increase Screen's memory allocation by 2x.

This patch converts FTXUI to use std::string internally. It now exposes
std::string based API. The std::wstring API remains, but is now
deprecated.

Tests and examples haven't been update to show the breakage is limited.
They will be updated in a second set of patches.

Bug: https://github.com/ArthurSonzogni/FTXUI/issues/153
Co-authored-by: Tushar Maheshwari <tushar27192@gmail.com>

include/ftxui/util/ref.hpp

@@ -42,11 +42,27 @@ class Ref {
 /// class convert multiple mutable string toward a shared representation.
 class StringRef {
  public:
-  StringRef(std::wstring* ref) : address_(ref) {}
-  StringRef(std::wstring ref) : owned_(std::move(ref)) {}
-  StringRef(const wchar_t* ref) : StringRef(std::wstring(ref)) {}
-  StringRef(const char* ref) : StringRef(to_wstring(std::string(ref))) {}
-  StringRef(std::string ref) : StringRef(to_wstring(std::move(ref))) {}
+  StringRef(std::string* ref) : address_(ref) {}
+  StringRef(std::string ref) : owned_(std::move(ref)) {}
+  StringRef(const wchar_t* ref) : StringRef(to_string(std::wstring(ref))) {}
+  StringRef(const char* ref) : StringRef(std::string(ref)) {}
+  std::string& operator*() { return address_ ? *address_ : owned_; }
+  std::string* operator->() { return address_ ? address_ : &owned_; }
+
+ private:
+  std::string owned_;
+  std::string* address_ = nullptr;
+};
+
+/// @brief An adapter. Own or reference a constant string. For convenience, this
+/// class convert multiple mutable string toward a shared representation.
+class WideStringRef {
+ public:
+  WideStringRef(std::wstring* ref) : address_(ref) {}
+  WideStringRef(std::wstring ref) : owned_(std::move(ref)) {}
+  WideStringRef(const wchar_t* ref) : WideStringRef(std::wstring(ref)) {}
+  WideStringRef(const char* ref)
+      : WideStringRef(to_wstring(std::string(ref))) {}
   std::wstring& operator*() { return address_ ? *address_ : owned_; }
   std::wstring* operator->() { return address_ ? address_ : &owned_; }
 
@@ -59,19 +75,35 @@ class StringRef {
 /// class convert multiple immutable string toward a shared representation.
 class ConstStringRef {
  public:
-  ConstStringRef(const std::wstring* ref) : address_(ref) {}
-  ConstStringRef(std::wstring ref) : owned_(std::move(ref)) {}
+  ConstStringRef(const std::string* ref) : address_(ref) {}
+  ConstStringRef(const std::wstring* ref) : ConstStringRef(to_string(*ref)) {}
+  ConstStringRef(std::string ref) : owned_(std::move(ref)) {}
+  ConstStringRef(std::wstring ref) : ConstStringRef(to_string(ref)) {}
   ConstStringRef(const wchar_t* ref) : ConstStringRef(std::wstring(ref)) {}
   ConstStringRef(const char* ref)
       : ConstStringRef(to_wstring(std::string(ref))) {}
-  ConstStringRef(std::string ref)
-      : ConstStringRef(to_wstring(std::move(ref))) {}
-  const std::wstring& operator*() { return address_ ? *address_ : owned_; }
-  const std::wstring* operator->() { return address_ ? address_ : &owned_; }
+  const std::string& operator*() { return address_ ? *address_ : owned_; }
+  const std::string* operator->() { return address_ ? address_ : &owned_; }
 
  private:
-  const std::wstring owned_;
-  const std::wstring* address_ = nullptr;
+  const std::string owned_;
+  const std::string* address_ = nullptr;
+};
+
+/// @brief An adapter. Reference a list of strings.
+class ConstStringListRef {
+ public:
+  ConstStringListRef(const std::vector<std::string>* ref) : ref_(ref) {}
+  ConstStringListRef(const std::vector<std::wstring>* ref) : ref_wide_(ref) {}
+
+  size_t size() const { return ref_ ? ref_->size() : ref_wide_->size(); }
+  std::string operator[](size_t i) const {
+    return ref_ ? (*ref_)[i] : to_string((*ref_wide_)[i]);
+  }
+
+ private:
+  const std::vector<std::string>* ref_ = nullptr;
+  const std::vector<std::wstring>* ref_wide_ = nullptr;
 };
 
 }  // namespace ftxui