vec needs clear

Similarly to #2775 -- FIXME in typeck::transaction_methods::commit says "could use a vec::clear() that ran destructors but kept the vec at its currently allocated length"

I think clear would be counterintuitive to the use in other languages (returning an empty vector/list/array). Maybe blank instead?

If I understand the issue correctly, vec::clear(v) can be expressed as vec::truncate(v, 0) as of ec9c68c.