MINOR: channel: add a few basic functions for the new buffer API

MINOR: channel: add a few basic functions for the new buffer API

This adds :
  - c_orig()  : channel buffer's origin
  - c_size()  : channel buffer's size
  - c_wrap()  : channel buffer's wrapping location
  - c_data()  : channel buffer's total data count
  - c_room()  : room left in channel buffer's
  - c_empty() : true if channel buffer is empty
  - c_full()  : true if channel buffer is full

  - c_ptr()   : pointer to an offset relative to input data in the buffer
  - c_adv()   : advances the channel's buffer (bytes become part of output)
  - c_rew()   : rewinds the channel's buffer (output bytes not output anymore)
  - c_realign_if_empty() : realigns the buffer if it's empty

  - co_data() : # of output data
  - co_head() : beginning of output data
  - co_tail() : end of output data
  - ci_data() : # of input data
  - ci_head() : beginning of input data
  - ci_tail() : end of input data
  - ci_stop() : location after ci_tail()
  - ci_next() : pointer to next input byte

And for the ci_* / co_* functions above, the "__*" variants which disable
wrapping checks, and the "_ofs" variants which return an offset relative to
the buffer's origin instead.