Structured commands are just Eshell flow control commands, like 'if' and 
'while'. These expected you to set 'eshell-test-body' and 
'eshell-command-body' when implementing them, and were very hard to get 
right (see bug#12571 for example).

Instead, let's improve Eshell's iterative command evaluation some more 
so that you can write the implementations for these commands like 
normal. I think this is beneficial for two main reasons: a) it should 
get us closer to replacing Eshell's iterative evaluation with the CPS 
machinery in generator.el (bug#37635)[1], and b) third-parties should 
have an easier time writing their own fancy Eshell commands using the 
command-rewriting hooks.

Given that we have fairly thorough regression tests in Eshell now, I'm 
pretty confident that these patches don't break anything.

[1] I think I mentioned it elsewhere, but I'm hoping to make sure that 
when we do switch to the CPS machinery (or maybe to threads), it's just 
a drop-in replacement. That way, if there are problems, it should be 
easy to revert to the classic iterative evaluation code. Maybe we could 
even provide a defcustom for it just in case.