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These indicate the size and time span of a sequence of events in the
event log, to make it easier to sort and navigate a large event log.

  - tracing facilities are now enabled with -DTRACING, and -DDEBUG
    additionally enables debug-tracing.  -DEVENTLOG has been
    removed.

  - -debug now implies -eventlog

  - events can be printed to stderr instead of being sent to the
    binary .eventlog file by adding +RTS -v (which is implied by the
    +RTS -Dx options).

  - -Dx debug messages can be sent to the binary .eventlog file
    by adding +RTS -l.  This should help debugging by reducing
    the impact of debug tracing on execution time.

  - Various debug messages that duplicated the information in events
    have been removed.

The first phase of this tidyup is focussed on the header files, and in
particular making sure we are exposinng publicly exactly what we need
to, and no more.

 - Rts.h now includes everything that the RTS exposes publicly,
   rather than a random subset of it.

 - Most of the public header files have moved into subdirectories, and
   many of them have been renamed.  But clients should not need to
   include any of the other headers directly, just #include the main
   public headers: Rts.h, HsFFI.h, RtsAPI.h.

 - All the headers needed for via-C compilation have moved into the
   stg subdirectory, which is self-contained.  Most of the headers for
   the rest of the RTS APIs have moved into the rts subdirectory.

 - I left MachDeps.h where it is, because it is so widely used in
   Haskell code.
 
 - I left a deprecated stub for RtsFlags.h in place.  The flag
   structures are now exposed by Rts.h.

 - Various internal APIs are no longer exposed by public header files.

 - Various bits of dead code and declarations have been removed

 - More gcc warnings are turned on, and the RTS code is more
   warning-clean.

 - More source files #include "PosixSource.h", and hence only use
   standard POSIX (1003.1c-1995) interfaces.

There is a lot more tidying up still to do, this is just the first
pass.  I also intend to standardise the names for external RTS APIs
(e.g use the rts_ prefix consistently), and declare the internal APIs
as hidden for shared libraries.

Also some tidyups and renaming

Fixed ThreadID to be defined as StgThreadID, not StgWord64.  Changed CapabilityNum to CapNo.  Added helper functions postCapNo() and postThreadID().
ThreadID was StgWord64, but should have been StgThreadID, which is
currently StgWord32.  Changed name from CapabilityNum to CapNo to better
reflect naming in Capability struct where "no" is the capability number.
Modified EventLog.c to use the helper functions postCapNo() and
postThreadID () for CapNo and ThreadID.

Generate binary log files from the RTS containing a log of runtime
events with timestamps.  The log file can be visualised in various
ways, for investigating runtime behaviour and debugging performance
problems.  See for example the forthcoming ThreadScope viewer.

New GHC option:

  -eventlog   (link-time option) Enables event logging.

  +RTS -l     (runtime option) Generates <prog>.eventlog with
              the binary event information.

This replaces some of the tracing machinery we already had in the RTS:
e.g. +RTS -vg  for GC tracing (we should do this using the new event
logging instead).

Event logging has almost no runtime cost when it isn't enabled, though
in the future we might add more fine-grained events and this might
change; hence having a link-time option and compiling a separate
version of the RTS for event logging.  There's a small runtime cost
for enabling event-logging, for most programs it shouldn't make much
difference.

(Todo: docs)