Change some things to string_view

src/support/json.h

diff --git a/src/support/json.h b/src/support/json.h
index 98e46b0..2d57fef 100644
--- a/src/support/json.h
+++ b/src/support/json.h
@@ -467,13 +467,12 @@
 
 private:
   // Unescape the input (UTF8) string into one of our internal strings (WTF16).
-  void unescapeIntoWTF16(char* str) {
+  void unescapeIntoWTF16(std::string_view str) {
     // TODO: Optimize the unescaped path? But it is impossible to avoid an
     //       allocation here.
     std::stringstream ss;
     wasm::String::unescapeUTF8JSONtoWTF16(ss, str);
-    // TODO: Use ss.view() once we have C++20.
-    setString(ss.str());
+    setString(ss.view());
   }
 };
 

src/support/string.cpp

diff --git a/src/support/string.cpp b/src/support/string.cpp
index d3fbcec..c410c70 100644
--- a/src/support/string.cpp
+++ b/src/support/string.cpp
@@ -499,19 +499,18 @@
   return true;
 }
 
-std::ostream& unescapeUTF8JSONtoWTF16(std::ostream& os, const char* str) {
-  std::string_view view(str);
+std::ostream& unescapeUTF8JSONtoWTF16(std::ostream& os, std::string_view str) {
   size_t i = 0;
   while (str[i]) {
     if (str[i] != '\\') {
       // Normal character, possibly a multi-byte UTF-8 sequence.
-      auto rest = view.substr(i);
+      auto rest = str.substr(i);
       auto u = takeWTF8CodePoint(rest);
       if (!u) {
         Fatal() << "Invalid UTF-8 in JSON string";
       }
       writeWTF16CodePoint(os, *u);
-      i = view.size() - rest.size();
+      i = str.size() - rest.size();
       continue;
     }
 
@@ -548,7 +547,7 @@
     if (!str[i + 2] || !str[i + 3] || !str[i + 4] || !str[i + 5]) {
       Fatal() << "Invalid escaped JSON \\uXXXX";
     }
-    unhex << std::hex << std::string_view(str + i + 2, 4);
+    unhex << std::hex << str.substr(i + 2, 4);
     unhex >> x;
 
     // Write out the results.

src/support/string.h

diff --git a/src/support/string.h b/src/support/string.h
index 4e9d108..6b7b6a7 100644
--- a/src/support/string.h
+++ b/src/support/string.h
@@ -128,7 +128,7 @@
 bool isPrintableUTF8(std::string_view str);
 
 // Given a string of properly-escaped JSON in UTF8, unescape it into WTF16.
-std::ostream& unescapeUTF8JSONtoWTF16(std::ostream& os, const char* str);
+std::ostream& unescapeUTF8JSONtoWTF16(std::ostream& os, std::string_view str);
 
 } // namespace wasm::String