TXR gets delimited continuations.

* unwind.c (sys_cont_s): New symbol variable.
(uw_pop_block): New function, supporting uw_block_end macro.
Special logic is needed for popping blocks now, because a
block can be a captured stub at the top of a continuation,
which must not be popped in the
regular manner.
(struct cont): New struct type.
(cont_destroy, cont_mark): New static functions.
(cont_ops): New static structure.
(revive_cont, capture_cont): New static functions.
(uw_capture_cont): New functions.
(uw_init): Initialize sys_cont_s.
Register sys:capture-cont intrinsic.

* unwind.h (enum uw_frtype): New enum member
UW_CAPTURED_BLOCK. When a block is captured as a stub copy of
the prompt block of a delimited continuation, its type is
changed from UW_BLOCK to this new type. This does two things:
it makes the block invisible to block-related code that does
nothing with continuations (code that looks for UW_BLOCK
and ignores anything else). Secondly, there is some special
handling related to UW_CAPTURED_BLOCK frames.
(uw_pop_block, uw_capture_cont): Declared.
(uw_block_begin): New local pointer uw_rslt introduced
which helps communicate the result variable over to the
uw_block_end macro (so we don't have to add a variable
argument to the latter).
(uw_block_end): Use uw_pop_block instead of uw_pop_frame.

* txr.1: Documented delimited continuations.

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