[wpiutil] StringExtras: Add substr() (#3742)

Unlike std::string and std::string_view, this substr() allows a start
greater than the length of the string, in which case an empty string
is returned.  This matches llvm::StringRef behavior.

wpiutil/src/main/native/cpp/HttpUtil.cpp

@@ -232,7 +232,7 @@ bool FindMultipartBoundary(raw_istream& is, std::string_view boundary,
 
     // Did we find the boundary?
     if (searchBuf[0] == '-' && searchBuf[1] == '-' &&
-        searchBuf.substr(2) == boundary) {
+        wpi::substr(searchBuf, 2) == boundary) {
       return true;
     }