use a new value for vectors in the IL

can't use CConv because they automatically get block_tag added to their
tags, and vector_tag is below block_tag. this way is probably cleaner
than implicitly shifting all the block tags.
start on adding VfromList as a primitive intLang operation

this means it will need to be implemented in bytecode (or machine code).
implementing it in IL itself feels like at the wrong level of
abstraction, since we don't have direct access to bytecode blocks.
update intLangExtra for addition of CVectorv values