We can now translate layers 49-56 using the self-hosted translator
(`translate` and `ntranslate`).
As a follow-up to commit 5404, the self-hosted translator is a little
more strict than the C++ translator in 3 places:
a) All .subx files must define a data segment.
b) All .subx files must define an `Entry` label.
c) All numbers must be in *lowercase* hex.
In all cases, where programs work with the C++ translator but violate
the self-hosted translator's assumptions, we must make sure we raise
errors rather than silently emit bad code.
