Fix string_to_ident when ident is not fresh
!660 (merged) re-implemented string_to_ident
but doesn't do a clear
in the tactic-in-term to make sure it handles an existing name.
Perennial is actually relying on string_to_ident
rather than intros_by_string
because the NamedProps.v library needs to freshen a name coming from a string, so I worked around this bug by re-implementing string_to_ident
in Perennial.