Remove forceful "using namespace arduino" from headers

First step to fix arduino/ArduinoCore-samd#606

Author: facchinm

Diff for: api/Print.h

@@ -93,4 +93,4 @@ class Print
 };
 
 }
-using namespace arduino;
+using arduino::Print;

Diff for: api/String.h

@@ -255,5 +255,8 @@ class StringSumHelper : public String
 
 } // namespace arduino
 
+using arduino::__FlashStringHelper;
+using arduino::String;
+
 #endif  // __cplusplus
 #endif  // __ARDUINO_STRINGS__

Diff for: api/Udp.h

@@ -87,6 +87,4 @@ class UDP : public Stream {
   uint8_t* rawIPAddress(IPAddress& addr) { return addr.raw_address(); };
 };
 
-}
-
-using namespace arduino;
+}