Provide String::indexOf for a char* needle

cores/esp8266/WString.cpp

@@ -617,6 +617,19 @@ int String::indexOf(char ch, unsigned int fromIndex) const {
     return temp - buffer();
 }
 
+int String::indexOf(const char *s2) const {
+    return indexOf(s2, 0);
+}
+
+int String::indexOf(const char *s2, unsigned int fromIndex) const {
+    if (fromIndex >= len())
+        return -1;
+    const char *found = strstr(buffer() + fromIndex, s2);
+    if (found == NULL)
+        return -1;
+    return found - buffer();
+}
+
 int String::indexOf(const String &s2) const {
     return indexOf(s2, 0);
 }

cores/esp8266/WString.h

@@ -235,6 +235,8 @@ class String {
         // search
         int indexOf(char ch) const;
         int indexOf(char ch, unsigned int fromIndex) const;
+        int indexOf(const char *str) const;
+        int indexOf(const char *str, unsigned int fromIndex) const;
         int indexOf(const String &str) const;
         int indexOf(const String &str, unsigned int fromIndex) const;
         int lastIndexOf(char ch) const;

tests/host/core/test_string.cpp

@@ -245,6 +245,8 @@ TEST_CASE("String nulls", "[core][String]")
     REQUIRE(s.lastIndexOf("tacos") == -1);
     REQUIRE(s.lastIndexOf('t', 0) == -1);
     REQUIRE(s.lastIndexOf('t') == -1);
+    REQUIRE(s.indexOf(String("tacos"), 1) == -1);
+    REQUIRE(s.indexOf(String("tacos")) == -1);
     REQUIRE(s.indexOf("tacos", 1) == -1);
     REQUIRE(s.indexOf("tacos") == -1);
     REQUIRE(s.indexOf('t', 1) == -1);