we'll use this in NarAccessor to provide actually safe indexing of
archives. NarAccessor currently is not fully correct: it relies on
the parser not buffering anything to produce correct file offsets,
but only the nar implementation itself can reasonably expect that.
Change-Id: I64f300b86d8844b876a3ace723546ea7d7b4628b
this is where they should've been from the start, but during the first
rewrite it made little sense to move them. we have bigger plans today,
so we'll finally clean that up too. note the `Map` transform type that
is needed to make the current macros work. it shall be only temporary.
Change-Id: I928d197dbfe27b68cf8634d149c3259e86fbf123
eldritch horrors