Fix reverting of local implicit funs in erl_syntax

This partially reverts 290dc2b08a2f92157ac5358fba815f4dbb32f8f2 in which
implicit funs were mistakenly thought to be using abstract terms for their name
and arity.
Smoke test erl_syntax:revert/1

This tries to make bugs like 87156887b7d82a2db53f60441a4af87034cb8789 not
happen again.

The new test takes each file in the stdlib app, passes their forms through the
erl_syntax representation, reverts them and compiles them.
Prepare release