Everywhere: Rename to_{string => deprecated_string}() where applicable

This will make it easier to support both string types at the same time
while we convert code, and tracking down remaining uses.

One big exception is Value::to_string() in LibJS, where the name is
dictated by the ToString AO.

Userland/Libraries/LibWasm/AbstractMachine/Validator.h

@@ -306,7 +306,7 @@ private:
                 }
             }
             builder.append(']');
-            return { builder.to_string() };
+            return { builder.to_deprecated_string() };
         }
 
     private:

Userland/Libraries/LibWasm/Parser/Parser.cpp

@@ -1364,7 +1364,7 @@ bool Module::populate_sections()
     return is_ok;
 }
 
-DeprecatedString parse_error_to_string(ParseError error)
+DeprecatedString parse_error_to_deprecated_string(ParseError error)
 {
     switch (error) {
     case ParseError::UnexpectedEof:

Userland/Libraries/LibWasm/Types.h

@@ -43,7 +43,7 @@ enum class ParseError {
     NotImplemented,
 };
 
-DeprecatedString parse_error_to_string(ParseError);
+DeprecatedString parse_error_to_deprecated_string(ParseError);
 
 template<typename T>
 using ParseResult = Result<T, ParseError>;