hecl/hecl: Slightly improve resource usage within string conv operator+ funcs

We already construct a std::string instance, so we can just append to
it instead of creating another temporary with std::string's operator+.
We also change this to append using the string view getter functions, as
this allows the appending process to do less work. When a pointer is
passed in, a strlen call would need to be performed in order to
determine the total characters to append. However, we already know the
size (via the string view).

hecl/include/hecl/hecl.hpp

@@ -115,7 +115,9 @@ public:
   const char* c_str() const { return m_utf8.c_str(); }
   std::string operator+(std::string_view other) const { return m_utf8 + other.data(); }
 };
-inline std::string operator+(std::string_view lhs, const SystemUTF8Conv& rhs) { return std::string(lhs) + rhs.c_str(); }
+inline std::string operator+(std::string_view lhs, const SystemUTF8Conv& rhs) {
+  return std::string(lhs).append(rhs.str());
+}
 class SystemStringConv {
   std::wstring m_sys;
 
@@ -126,7 +128,7 @@ public:
   std::wstring operator+(const std::wstring_view other) const { return m_sys + other.data(); }
 };
 inline std::wstring operator+(std::wstring_view lhs, const SystemStringConv& rhs) {
-  return std::wstring(lhs) + rhs.c_str();
+  return std::wstring(lhs).append(rhs.sys_str());
 }
 #else
 class SystemUTF8Conv {
@@ -136,9 +138,11 @@ public:
   explicit SystemUTF8Conv(SystemStringView str) : m_utf8(str) {}
   std::string_view str() const { return m_utf8; }
   const char* c_str() const { return m_utf8.data(); }
-  std::string operator+(std::string_view other) const { return std::string(m_utf8) + other.data(); }
+  std::string operator+(std::string_view other) const { return std::string(m_utf8).append(other); }
 };
-inline std::string operator+(std::string_view lhs, const SystemUTF8Conv& rhs) { return std::string(lhs) + rhs.c_str(); }
+inline std::string operator+(std::string_view lhs, const SystemUTF8Conv& rhs) {
+  return std::string(lhs).append(rhs.str());
+}
 class SystemStringConv {
   std::string_view m_sys;
 
@@ -146,10 +150,10 @@ public:
   explicit SystemStringConv(std::string_view str) : m_sys(str) {}
   SystemStringView sys_str() const { return m_sys; }
   const SystemChar* c_str() const { return m_sys.data(); }
-  std::string operator+(std::string_view other) const { return std::string(m_sys) + other.data(); }
+  std::string operator+(std::string_view other) const { return std::string(m_sys).append(other); }
 };
 inline std::string operator+(std::string_view lhs, const SystemStringConv& rhs) {
-  return std::string(lhs) + rhs.c_str();
+  return std::string(lhs).append(rhs.sys_str());
 }
 #endif