make assertion error more meaningful/obvious.

i have a feeling i'll hit this in 6 months and not remember what
it means, then.
add unhandled R* register cases when reading from registers.
debug: improve an error message.
neither C nor Fortran lets you use "." in a symbol name.

Yet such symbols still appear: they are special linker or runtime
symbols.  Thus, we never care about accesses or allocations inside
them.