Simplify usage of 'while' forms in Eshell's iterative evaluation

Simplify usage of 'while' forms in Eshell's iterative evaluation

Now, 'eshell-do-eval' rewrites 'while' forms to let-bind variables for
the command and test bodies.  This means that external code, such as
command rewriting hooks, no longer has to worry about this, making it
easier to pass "normal" Lisp forms to 'eshell-do-eval' (bug#61954).

* lisp/eshell/esh-cmd.el (eshell-command-body, eshell-test-body): No
longer used outside of 'eshell-do-eval', so rename to...
(eshell--command-body, eshell--test-body): ... these.
(Command evaluation macros): Remove obsolete description about 'if'
and 'while' forms.
(eshell-rewrite-for-command, eshell-structure-basic-command): Remove
'eshell-command-body' and 'eshell-test-body'.
(eshell-do-eval): Reimplement handling of 'while' forms.