Immutable merge

[g105b]

This is a backwards breaking change, so will require implementation in two steps:

1. Introduce withMerge function, leaving merge as a mutable function, but start to emit a deprecation notice.
2. Upon next major release, remove merge function completely.

It might actually make more sense to make merge immutable by default, but due to this decision being made, withMerge is so much more obvious what it does due to the consistency with other with functions.