MINOR: buffer: add functions to read/write varints from/to buffers

MINOR: buffer: add functions to read/write varints from/to buffers

The new functions are :
  __b_put_varint() : inserts a varint when it's known that it fits
  b_put_varint()   : tries to insert a varint at the tail
  b_get_varint()   : tries to get a varint from the head
  b_peek_varint()  : tries to peek a varint at a specific offset

Wrapping is supported so that they are expected to be safe to use to
manipulate varints with buffers anywhere.