Add methods for appending Buildable objects to buffer.

[jtt]

Adds two methods, append() and append_all() to Builder. append() allows to add a Buildable to the end of buffer and append_all() can be used to append a slice of Buildables to the end of the buffer. Both methods will call the build() method of buildable to write data to buffer. If error occurs while appending data, the index of buffer is reset back to where it was when append() or append_all() was called.

Also add a new error type Error::InvalidData which allows the build() method of Buildable to signal that data was invalid and could not be written into buffer.