Please, organize the `Expr` module

[Anton-Latukha]

After #842, the next (previous) core part of the project is the Expr. It has already has quite a good coverage of documentation, but the module is ~715 LOCs:
Any of:

• The organizational structure there obviously needed. Look at Value module and organize the Expr.
• It may be documented along the way
• Hook-it-up that to the Wiki design article.
• And maybe Suggested improvements to NExpr #377. Disumburgate VarName etc..