Cost of logging and Chrono

[craff]

In function which are very frequently called, log and chrono can cost a lot.
If I uncomment the log in add_term and add_valu, the cost of log+chorno in Equiv and
Compare is a factor between 2 and 3.

we should create a binary "main.native" where this log/chrono on small function are disabled
(keeping still the main one like typing) and a main.debug with all log and chrono.

[craff]

This could be cleanly implemented having two functions per log and two functions Chrono.add_time,
the new one being inactive in main.native and active in main.debug.

We must make sure these function doing nothing are inlined and eliminated by OCaml.