Begin the "immutable revolution" by causing all finally merged component options to be immutable

This change has been trailled informally for a while, but it is time to break compatibility in a significant way by causing all finally merged options to be immutable. This is essential in order to begin the work for FLUID-6143 - the "memoised merged defaults stencil" object will be structured as an "immutable skeleton" on top of which the elements of the expanded, fetched material from around the tree will be "painted on" by means of the "liana-like" scheme described in https://wiki.fluidproject.org/display/fluid/The+State+of+Options+Merging+25-4-16 . This means that users will only pay the costs for instantiating the parts of the options structure that are actually variable between component instances, which should lead to huge economies both in reduced garbage and reduced instantiation time.