Rather than an "all or nothing" approach to defining a custom conversion
function (which seems destined to cause problems eventually), this is an
attempt to make it possible to call the auto-generated code and then "fix it
up".

Specifically, consider you have a fooBar struct.  If you don't define a
conversion for FooBar, you will get a generated function like:
    convert_v1_FooBar_To_api_FooBar()

Before this PR, if you define your own conversion function, you get no
generated function.  After this PR you get:
    autoconvert_v1_FooBar_To_api_FooBar()
...which you can call yourself in your custom function.