Share normalization of defined variables ?

[fblanqui]

Currently, normalization of defined variables is not shared: https://github.com/Deducteam/lambdapi/blob/326d987306c51f1092d5cef6c36fc28f26896aa4/src/core/eval.ml#L164-L168

In addition, would it be better to deal with let's by extending c.defmap instead? https://github.com/Deducteam/lambdapi/blob/326d987306c51f1092d5cef6c36fc28f26896aa4/src/core/eval.ml#L154-L155