compiler: block escape list.

The new dynamic variable *esc-blocks* keeps track of
what blocks, in a given scope, may be abandoned by
a simple jmp instruction instead of a full blown
dynamic return. We will not try to handle unwinding
statically; any contour that needs unwinding cannot
be jumped across.

* stdlib/compiler.tl (*esc-blocks*): New special variable.
(compile-in-top-level): Clear *esc-blocks* for
top-level compilations.
(compiler (comp-unwind-protect, comp-catch,
comp-lambda-impl, comp-prof): These contexts cannot be
abandoned by a jmp instruction: we cannot jump out of
the middle of an unwind-protect, catch, lambda or prof.
So we compile these with *esc-blocks* set to nil.
New blocks entirely contained in these constructs can
of course build up the list locally. E.g. within a function,
of course we can have blocks that are abandoned by a
simple jmp instruction. Just we cannot jump out.
(compiler comp-block): When compiling a block, we bind
*esc-blocks* to a list consisting of the previous value,
plus the new block name consed to the front.

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