cascade: Support deep-merging params into existing nested maps

I ran into this while working on the docs site, and was surprised it was not possible:

project config:

[[cascade]]
  [cascade.params.functions_and_methods]
    type = 'function'
  [cascade.target]
    path = '/functions/**'
    kind = 'page'

The target pages already define the .params.functions_and_methods map, for example:

params:
  functions_and_methods:
    aliases: [float]
    returnType: float64
    signatures: [cast.ToFloat INPUT]

This is about deep-merging maps, not slices, and is a low-priority change -- there are many others ways to do what I need to do.