Introduce IR.FuncE (AST-26)

[nomeata]

this turned out to be a bigger change, uncovering a few dirty spots in
Check_ir and other places.

This codifies also that, in the IR:

• All BlockE expressions end with an ExprE
• Only ExpD declarations return their type (this is differs from
  Source)

[nomeata]

Hmpf, this branch name confuses Jenkins. Let’s try again.