Fixing part of HoTT bug #61 (in declare_evar_from_virtual_equation,
fix the type of the term which has to be in the signature of the evar
to declare); some problems remain though (see next commit).
Unification in HoTT_coq_061.v was looping with previous commit (while
it was failing with Not_found before previous commit). This "fixes"
the loop by expanding local defs in "imitate" rather than keeping them
explicit. The example is otherwise too large for me to be able to
understand where does the loop come from.