cl-loop extensibility is impossible with current documented interfaces

[Daniel Colascione]

cl-loop defines cl-loop-for-handler and cl-loop-handler as public
interfaces for extending the set of keywords that loop supports.
There is no public interface, however, for doing anything useful in
the functions these handlers describe.  We should document
cl--loop-args and, for cl-loop-for-handler, loop-for-bindings,
loop-for-sets, and loop-for-steps so that loop extenders don't have
to understand the cl-macs source to understand what's going on.
Additionally, we should *dynamically* bind loop-for-bindings,
loop-for-sets, and loop-for-steps so that handler functions can
actually modify these variables as they used to be able to do before
lexbind.

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