Traverse `op` property when it's an `Identifier`

[k4b7]

The math AST uses Identifier nodes to represent named functions, e.g. sin(x) is parsed as:

{
  type: 'Apply',
  op: { type: 'Identifier', name: 'sin' },
  args: [ { type: 'Identifier', name: 'x' } ],
}