Order map and struct keys

[angelikatyborska]

The order in which map and struct keys are defined shouldn't matter. We could order them alphabetically.

For example:

Input:

%Time{
  hour: 1,
  minute: 30,
  seconds: 0,
  microsecond: {0, 0},
  calendar: Calendar.ISO
}

Desired representation:

%Time{
  calendar: Calendar.ISO,
  hour: 1,
  microsecond: {0, 0},
  minute: 30,
  seconds: 0
}

Important: don't do this for keyword lists.

The issue of whether keys will be replaced by placeholders is still open, so make sure to sort the keys before they are replaced (if they will get replaced).