Fixing bug #2900 (evar/evar unif was supposed to be treated in
solve_simple_eqn but in case the second evar was hidden behind a
local variable, it arrived in evar_define and imitate, wrongly
assuming progress).
Propagating conversion_problem towards (postponed) evar/evar problems.
Incidentally simplifies where evar/evar problems are treated (in
evar_define and imitate rather than solve_simple_eqn).
Updated debugging printers