vm: restore the VM profiler

[zerbina]

Summary

Fix the compiler crashing when enabling the VM profiler (--profilevm)
and fully restore the latter's original functionality. In addition, the
profiler's overhead is significantly reduced.

Details

The VM profiler stopped working when the stack-frames moved from being
ref based to being seq based (fb03691).

This is due to the profiler expecting the frame index specified at the
start of a measurement (enter) still being valid at the end of a
measurement (leave), but this is not true when executing, for example,
a Ret instruction, which led to an out-of-bounds error.

As a simple and low impact fix, instead of remembering the index,
leave now takes the list of current stack frames (i.e., the stack) as
input, treating the last item as the current frame. A minor side-effect
of this is that a Ret instruction now counts for the caller, while a
call instruction counts for the callee.

There was also a bug in the rendering logic: min was used instead of
max, meaning that the entry limiting didn't work.

In addition, the documentation of the vmprofiler module is improved
and the implementation is refactored:

• the data table now uses a PSym as the key. The intention of the
  table was always to associate data with a procedure, but TLineInfo
  and later SourceLinePosition only approximated this. Using a PSym
  also makes it possible to render more information (such as the
  procedure's name) in the future
• whether profiling is enabled is now a property of the Profiler
  instance, removing the dependency on a ConfigRef. This also slightly
  reduces the overhead of enter/leave
• the leaveImpl procedure is improved. The time difference is only
  computed once, and instead of temporarily assigning each frame to a
  local (which incurred a very costly full copy), only prc is assigned
  to a local

With access to the symbol, rendering now also renders the full file,
line, and column of the profiled procedures again (instead of only the
containing file).

Finally, an integration test (tprofiler.nim) and a basic unit test
(tvmprofiler.nim) are added in order to prevent future regressions.

[saem]

i'm looking forward to folks using this to debug various issues

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• follows up to internal: separate VM profiler data from ConfigRef #899 and vmprofiler: use Table instead of TableRef #904
• an additional efficiency improvement is possible, which I'm going implement after this PR is merged