Create mapping coq evars -> elpi evar (to avoid typechecking while solving)

[FissoreD]

1. Given a goal $G$ with the list of evars $c_1...c_n$, build a list of elpi fresh variables $e_1...e_n$.
2. The goal to analyse is $G'$ where each $c_i$ is replaced by $e_i$.
3. Launch resolution on $G'$.
4. When a solution is found, map back each $e_i$ to $c_i$

This variable replacement avoid the trigger of coq suspended evar.