Add evaluation tracing code

This patch is based on Justin Blanchard's original
work and allows to breakdown evaluation in its sub terms and
show to the user.

Tracing code has zero speed impact when not used.

Note that tracing code is not thread-safe, but this
should not be a problem given the typical usage scenario.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>