Add eqWAlizer analyses and reporting

Summary:
- Deprecate ad-hoc analysis of escape hatches
- Unified framework to write analyses in crate eqWAlizer, powered by `elp eqwalizer-stats`
- Implement analysis of escape hatches using visitor trait

Reviewed By: alanz

Differential Revision: D54063805

fbshipit-source-id: 3559f955074910877dee6a7eb8ff5a3dc5f2db6d

docs/reference/stats.md

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+# eqWAlizer stats
+
+This page references stats that eqWAlizer can report using command `elp eqwalize_stats`.
+
+### eqwalizer_fixme
+
+This indicates the suppression of an error message using a `%eqwalizer:fixme` escape hatch.
+This is discouraged and should be avoided at all costs.
+
+### eqwalizer_ignore
+
+This indicates the suppression of an error message using a `%eqwalizer:ignore` escape hatch.
+This is discouraged and should be avoided at all costs.
+
+### eqwalizer_nowarn
+
+This indicates the suppression of all eqWAlizer errors associated to a function,
+using a `-eqwalizer({nowarn_function, foo/n})` pragma.
+eqWAlizer will not run on such a function, thus the use of this pragma is highly
+discouraged.