Fix incorrect definition of FPSTR() and move from pgmspace.h to WString.h (#1371) (#2961)

Author: bxparks

cores/esp32/WString.h

@@ -35,7 +35,8 @@ class StringSumHelper;
 // an abstract class used as a means to proide a unique pointer type
 // but really has no body
 class __FlashStringHelper;
-#define F(string_literal) (reinterpret_cast<const __FlashStringHelper *>(PSTR(string_literal)))
+#define FPSTR(pstr_pointer) (reinterpret_cast<const __FlashStringHelper *>(pstr_pointer))
+#define F(string_literal) (FPSTR(PSTR(string_literal)))
 
 // The string class
 class String {

cores/esp32/pgmspace.h

@@ -32,7 +32,6 @@ typedef unsigned long prog_uint32_t;
 #define PROGMEM
 #define PGM_P         const char *
 #define PGM_VOID_P    const void *
-#define FPSTR(p)      ((const char *)(p))
 #define PSTR(s)       (s)
 #define _SFR_BYTE(n)  (n)