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Make it [expand_symbol] the main entry point instead of [expand_rule].

Use it to simplify [find_inlining_site].
Also, simplify the result of [find_inlining_site].

Fix another bug in [Action.rename], where a cascade of [let/in] was
used instead of a single [let/and], causing a capture.
Introduce a better renaming convention, using a suffix _inlined['0'-'9']+
instead of accumulating "'" suffixes.

which shows that inlining is currently broken (six tests fail).