While the existing _forEachField in ReflectionMirror.swift
already gives the offsets and types for each field, this isn't
enough information to construct a keypath for that field in
order to modify it.
For reference, this should be sufficent to implement the features
described here: (https://forums.swift.org/t/storedpropertyiterable/19218/62)
purely at runtime without any derived conformances for many types.
Note: Since there isn't enough reflection information for
`.mutatingGetSet` fields, this means that we're not able to support
reflecting certain types of fields (functions, nonfinal class fields,
etc). Whether this is an error or not is controlled by the `.ignoreUnknown`
option.
