`setExtensionLazy`
Rationale
* sorts well with `setExtension` for tooling
* implies that it is related to `setExtension` in some way
* at-a-glance disambiguates it from `setExtension` with a clue as
to how it is differrent
The disadvantage is that its a little long, but that's due to the
length of "Extension" not the "Lazy" suffix. If it's a problem
simply create `setExt` and `setExtLazy` and have `setExtension`
forward to `setExt`for backward compatibility with a friendly
comment. It wouldn't even need to be deprecated.
Mike
