Currently the 'MetricExpr' json value is passed from the json
file to the pmu-events.c. This change introduces an expression
tree that is parsed into. The parsing is done largely by using
operator overloading and python's 'eval' function. Two advantages
in doing this are:

1) Broken metrics fail at compile time rather than relying on
   `perf test` to detect. `perf test` remains relevant for checking
   event encoding and actual metric use.

2) The conversion to a string from the tree can minimize the metric's
   string size, for example, preferring 1e6 over 1000000, avoiding
   multiplication by 1 and removing unnecessary whitespace. On x86
   this reduces the string size by 2,930bytes (0.07%).

In future changes it would be possible to programmatically
generate the json expressions (a single line of text and so a
pain to write manually) for an architecture using the expression
tree. This could avoid copy-pasting metrics for all architecture
variants.

v4. Doesn't simplify "0*SLOTS" to 0, as the pattern is used to fix
    Intel metrics with topdown events.
v3. Avoids generic types on standard types like set that aren't
    supported until Python 3.9, fixing an issue with Python 3.6
    reported-by John Garry. v3 also fixes minor pylint issues and adds
    a call to Simplify on the read expression tree.
v2. Improvements to type information.

Committer notes:

Added one-line fixer from Ian, see first Link: tag below.
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Sumanth Korikkar <sumanthk@linux.ibm.com>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Link: https://lore.kernel.org/r/CAP-5=fWa=zNK_ecpWGoGggHCQx7z-oW0eGMQf19Maywg0QK=4g@mail.gmail.com
Link: https://lore.kernel.org/r/20221207055908.1385448-1-irogers@google.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>