Convert string buffers to use String class

[matthijskooijman]

A lot of places now use dynamic buffers using a lot of reallocing. We should replace this by instances of Arduino's String class, because:

• That simplifies code
• That keeps an explicit length, saving a lot of strlen() calls we have now
• Reduces chances of getting memory leaks

However, to do this efficiently, we should first finish billroy/bitlash#31 which makes it easier to let bitlash print to a String (by having a StringStream class for example).

When implementing this, we should consider to use the String::reserve method to allocate memory in blocks instead of reallocing for every single character added (e.g., we should call reserve((new_len + BLOCK_SIZE - 1) % BLOCK_SIZE) before adding to the string.

Furthermore, after we're done with a buffer, we should assign NULL to it buffer = NULL, which will be handled by an overloaded operator= to empty the buffer and free the memory (seems this is currently the only way to shrink the memory).