cleanup writeString()

[hmueller01]

Removed writeString(const char* string, uint8_t* buf, size_t pos) by using writeString(const char* string, uint8_t* buf, size_t pos, size_t size) all over.

This reduces stack consumption and avoids potential errors if called with another buffer that this->buffer.

[TD-er]

writeString is a private member function and every call made to it is given the member variable of the buffer size.
So why not just use the member variable within that function?

[hmueller01]

ATM always this->buffer is used, so this->buffer_size is always the max length. So yes, the member could be stripped by the two variables ...
@TD-er Do you need it for your “new" buffer algorithm? BTW, when does you PR come? ;-)