Use 'const' in 'buffer_size', 'buffer_remaining'

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Use 'const' in 'buffer_peek_8', 'buffer_peek_le16'

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Use 'const' in 'buffer_start', 'buffer_end'

Signed-off-by: Jussi Virtanen <jussi.k.virtanen@gmail.com>
Merge pull request #31 from jvirtanen/cleanups/buffer-const

Use 'const' in 'struct buffer' methods