Improve pushback behavior

[sliekens]

Being able to backtrack within a text source is an important requirement for most parsing algorithms.

Backtracking is currently implemented like this:

• use Stream.Seek(offset, SeekOrigin.Current) for any stream that support seeking
• use an instance of the PushbackInputStream class for streams that do not support seeking
  • The PushbackInputStream class uses internal byte buffers to store pushed-back bytes

Right now, the TextScanner class implements both behaviors using a complex soup of if..else blocks to determine which of the two strategies to use.

Instead, I want to split the TextScanner class into two classes, where each class implements one of the two backtracking strategies:

• class TextScanner will implement the seeking strategy
• class BufferedTextScanner will implement the pushback strategy