Merge topic branch t/btr into master

Some helpers such as btr_get_input_queue_size() are very expensive
if the buffer tree contains many buffers, which is the case for mp3
and aac streams. This is noticeable in particular with para_play,
which spends more time in the btr subsystem than in the mp3 decoder
if the file is large enough. This branch contains a few patches to
improve that.

Cooking for four months.

* refs/heads/t/btr:
  btr: Streamline the documentation of buffer_tree.h.
  Constify buffer tree API.
  btr: Merge buffers on insertion.
  btr: Speed up btr_node_status().
  Speed up mp3dec filter.

diff --cc NEWS.md
index 0f7e0c84e05946bab66b3dbd5acab11c9c6935d5,d5812289640d71c52a60bd850928c72b3c5ba0b8..416f99c66894badcd781d46e8e31b8a5e544c0f8
--- 1/NEWS.md
--- 2/NEWS.md
+++ b/NEWS.md
@@@ -5,10 -5,6 +5,13 @@@ NEW
  0.7.4 (to be announced) "genetic contraction"
  ---------------------------------------------
  
 +- Simplification of the build system. The only visible effect is
 +  that configure no longer prints a summary at the end.
 +- para_play is no longer built if libreadline is not installed.
++- Speedups for the buffer tree code, which particularly improves the
++  performance of para_play for large mp3 mp4 files.
++
 +
  Downloads:
  [tarball](./releases/paraslash-git.tar.xz)