Rename a few grammar wide variables.

Partly for clarity, and partly because there are too many things called
'block' and 'phrase'. The grammar names aren't great, but at least they
match. Might rething once I've done a lot of the 1.7 things.
Rename some local variables.

Just to reduce confusion between the paragraph rule and the paragraph
action. Should possibly do this for all such local variables. I'll see
how it goes.
Better element handling in `syntactic_block_item`.

Now all elements are handled within local.common.
Error if element used in incorrect context.

Should be less suprising than just not processing them. Needs more tests.
Error if phrase element body doesn't parse.