Initial implementation of SynchronousBuffer

Work on SynchronousBuffer should allow the parser objects to be called
from a single thread, in a stream-oriented fashion (similar to the
existing libwebvtt).
Implement StringBuilder::append()

StringBuilder::append() and other related routines for
growing the buffer (and calculating the buffer size to
allocate) have been added to the library.
Remove unneeded 'Detail' subdirectory

The IncrementalBufferDetail class has been removed,
and the Detail directory is no longer needed, because
of this.
Implemented SynchronousBuffer object, version 1