Fix some things in istring

src/support/istring.h

diff --git a/src/support/istring.h b/src/support/istring.h
index dad20b8..162a960 100644
--- a/src/support/istring.h
+++ b/src/support/istring.h
@@ -36,9 +36,11 @@
   static std::string_view interned(std::string_view s, bool reuse = true);
 
 public:
-  const std::string_view str;
+  std::string_view str;
 
   IString() = default;
+  IString(const IString& other) = default;
+  IString& operator=(const IString& other) = default;
 
   // TODO: This is a wildly unsafe default inherited from the previous
   // implementation. Change it?
@@ -49,12 +51,6 @@
   IString(const char* str) : str(interned(str, false)) {}
   IString(const std::string& str) : str(interned(str, false)) {}
 
-  IString(const IString& other) = default;
-
-  IString& operator=(const IString& other) {
-    return *(new (this) IString(other));
-  }
-
   bool operator==(const IString& other) const {
     // Fast! No need to compare contents due to interning
     return str.data() == other.str.data();