product/arithmetic each: missing block nil.

* stdlib/arith-each.tl (sys:vars-check): New function, copy
and pasted from each-prod.tl.
(sys:arith-each): New macro.
(sum-each, sum-each*, mul-each, mul-each*): Reworked using
sys:arith-each macro.  This macro uses logic borrowed from
a stripped-down expand-each in the compiler.

* stdlib/each-prod.tl (sys:expand-each-prod,
sys:expand-arith-each-prod*): Add the block nil around the
mapping call, taking care that the initialization forms
are evaluated outside of the block, and their values bound to
gensyms that then form the function arguments.

* txr.1: Document the missing requirements for all the
affected macros that there must be an anonymous block around
the body, which, if used, determines the return value.

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