Make Print and Stream constructors constexpr.

teensy3/Print.h

 class Print
 {
   public:
-	Print() : write_error(0) {}
+	constexpr Print() : write_error(0) {}
 	virtual size_t write(uint8_t b) = 0;
 	size_t write(const char *str)			{ return write((const uint8_t *)str, strlen(str)); }
 	virtual size_t write(const uint8_t *buffer, size_t size);

teensy3/Stream.h

 class Stream : public Print
 {
   public:
-	Stream() : _timeout(1000), read_error(0) {}
+	constexpr Stream() : _timeout(1000), read_error(0) {}
 	virtual int available() = 0;
 	virtual int read() = 0;
 	virtual int peek() = 0;