Add ability to loop though dictionaries

[ftruter]

Looping through arrays is already possible via the #for( something in array ): syntax. But I often need to loop through dictionaries. In my projects I often have [UUID: Codable] or [String: Codable] structures.

I would like to be able to write

  struct Person : Codable {
    var name: String
    // ...more props
  }
  let persons : [String: Person] = [
    "a": Person(name: "tanner"), 
    "b": Person(name: "ziz"), 
    "c": Person(name: "vapor")
  ]
  let template = """
    #for(person in persons):
        #(key): #(person.name) index=#(index) last=#(isLast) first=#(isFirst)
    #endfor
  """
  render(template, ["persons": persons])

Should render

  a: tanner index=0 last=false first=true
  b: ziz index=1 last=false first=false
  c: vapor index=2 last=true first=false

Leaf's current #for implementation already exposes additional local in-loop variables index, isFirst and 'isLastand my proposal adds just one morekey` variable.

Alternatively extending the loop syntax to #for( (key, person) in persons ) would have provided control over the name of the in-loop variables, and this feels more "Swift-like" yet breaks with Leaf-kit tradition.

Another alternative was to loop through the keys instead of the values, e.g. #for( key in persons ): but this will require repeated dictionary lookups when interpolating values, e.g.

  #for(key in persons):
    #(key): #(persons[key].name) #(persons[key].address) #(persons[key].tel)
  #endfor

This option feels more like Javascript or Python but the repeated dictionary looks are tedious to write and has a runtime performance penalty.