Fail early when writing to a fake file runs out of space. Makes debugging
tests easier.
Reads from files, on the other hand, are only buffering to a temporary
stream, so it makes sense to silently stop when they run out of space.
In the process I uncovered a testing bug in pack.subx: I was missing a
trailing space in the expected result, but the test still passed because
the space was getting truncated. Being principled about aborting on overflow
by default will help avoid such issues.
Considering how much trouble a merge phase would be (commit 4978), it seems
simpler to just add the extra syntax for controlling the entry point of
the generated ELF binary.
But I wouldn't have noticed this if I hadn't taken the time to write out
the commit messages of 4976 and 4978.
Even if we happened to already have linked list primitives built, this
may still be a good idea considering that I'm saving quite a lot of code
in duplicated entrypoints.