Support horizontal @(block), phase 2.

Accepts produce a return value which is a vector object
carrying both vertical (data pointer + line number)
and horizontal context (position within line).
Both a vertical and horizontal @(block) construct look
at this and behave in some intelligent way.

* match.c (match_line_ctx): New member, data. Thus,
horizontal contexts now know the vertical list from whcih they
are derived.
(ml_all): Take data argument, and initialize new member.
(h_block): If a vector object emerges from the block, that
means the block terminated due to an accept. If the accept
is from a different line, or from vertical context, then
just keep whatever horizontal position is current in the
horizontal context. (We have no way of knowing how far we
advanced between the start of the block and the elem which
triggered the accept.) If the accept is from the same line
then advance to the indicated position.
(h_accept_fail): Produce a three-element vector object for the
accept long return value. This carries the bindings, the
vertical-style result value, and the horizontal position.
(freeform_prepare): Update ml_all call to include c->data.
(v_block): Like in h_block, handle a vector result value.
An accept emanating from horizontal context for the current
line causes the vertical context to advance to the next line.
Horizontal accept from a different line doesn't advance the
data. Accept from a vertical context behaves as before.
(v_accept_fail): Produce vector object for accept.
(v_trailer): When intercepting accept, patch new vector
representation of accept value.
(match_files): Pass c.data to ml_all.

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