Expressions as traces

[rolyp]

Implementation tasks:

• "instantiate" (nee "translate")
• eval should no longer construct an evaluand, but get one from the input expression
• sort out trie type signatures/mess with trie nesting for products:
  • ConstrTrie should map constructor names to (T | Trie), not T
  • parser to enforce that constructors respect constructor signatures
  • parser to enforce that constructor patterns respect constructor signatures
  • evaluation and primitive evaluation can assume constructor signatures respected
• problem with Lex.Var not being versioned/interned (related to _shallowMergeAssign: recurse into non-versioned objects #22)