Add LetRec to JuvixCore

[lukaszcz]

Depends on PR #1421.

LetRec would be useful to implement "where" blocks and local/anonymous recursive functions.

A LetRec node would specify a block of local mutually recursive definitions:

letrec { 
  f1 := A1;
  f2 := A2;
  ...
  fn := An;
} in B

where all of f1, ..., fn may appear recursively in A1, ..., An

LetRec Node definition:

LetRec {letrecInfo :: Info, letrecValues :: NonEmpty Node, letrecBody :: Node}