Improve alet macro.

The alet macro should always convert bindings to constants
into symbol macros; the all-or-nothing logic should be applied
to any remaining bindings.

* share/txr/stdlib/place.tl (sys:r-s-let-expander): Generalize
this function somewhat more by passing in the fallback binding
symbol to use for bindings that can't be turned into symbol
macros, instead of hard-coding them to let.
(rlset, slet): Specify 'let when calling sys:r-s-let-expander.
(alet): If there are any bindings with constantp init
expressions, then recurse: produce an expansion which
separates constantp from non-constantp using
sys:r-s-let-expander. Pass 'alet as fallback binding symbol;
thus the expansion will recurse back to alet, but without
all the constantp bindings, if there are any.
We then deal with those using the existing all-or-nothing
logic (which simplifies slightly since it doesn't have
to check for constantp any more).

* txr.1: Revised description of alet.

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