Memo spec proposal

[kurtbrose]

Here's the proposal: Memo(), a glom memoization primitive.

On each top-level glom execution, Memo() would reset.

Here's a rough implementation:

class Memo:
   def __init__(self, spec):
      self.spec = spec

   def glom(self, target, scope):
      glom_globals = scope[ROOT]
      if self not in glom_globals:
         glom_globals[self] = ({}, {})
      val_memo, id_memo = glom_globals[self]
      try:
         hash(target)
      except:
         memo, memo_key = id_memo, id(target)
      else:
         memo, memo_key = val_memo, target
      if memo_key not in memo:
         memo[memo_key] = scope[glom](target, self.spec, scope)
      return memo[memo_key]

Motivation:

• allow for safe traversal of self-recursive data structures -- Memo(Ref(Match(Switch(...)))) would be able to safely unzip a into a self-recursive data structure -- memoization would result in a self-recursive output of a similar shape to the input

[kurtbrose]

also, kind of a dupe of #186