constantp: fully expand; recognize functions.

This patch improves the constantp function dramatically.  It
now performs a full expansion of its argument, and recognizes
all of the constant foldable functions that the compiler
recognizes.

* eval.c (const_foldable_s): New symbol variable.
(const_foldable_hash): New static variable.
(constantp_noex): Look up function in the hash table of const
foldable functions, including in the case when it appears in a
dwim form as in [+ 2 2] which is (dwim + 2 2). In this case,
recursively check the arguments for constantp_noex.
We get the hash table of foldable functions from the
sys:%const-foldable% variable, which comes from an autoloaded
module.
(constantp): Fully expand the input form, not just m
macroexpand.
(eval_init): Register the const_foldable_s variable.

* lisplib.c (constfun_instantiate, constfun_set_entries): New
static functions.
(lisplib_init): Register auto-loading of constfun module
via new static functions.

* stdlib/compiler.tl; Load the constfun module if
%const-foldable% is not defined.
(%const-foldable-funs%, %const-foldable%): Removed from here.

* stdlib/constfun.tl: New file.
(%const-foldable-funs%, %const-foldable%): Moved here.

* txr.1: Documented changes to constantp.

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