It appears that for some users `static-if' expands to nothing if
the THEN or ELSE part is a `defalias' form (whichever is used in
their case).  I got similar reports when using `static-if' more
than once, last in https://github.com/magit/magit/issues/5557,
and while I was never able to reproduce, I concluded that these
report are credible.

Closes #417

The built-in `if-let' et al. implementations are no longer used.

Shortly after this was added to Transient, `string-pixel-width'
was added to Emacs.  Also rearrange a bit to use same nesting as
was originally used in that function.

Make sure transient state is exited properly (except that the window
may remain visible of course).  We already demoted such errors, in one
place in this function, where we expected such misconfigurations could
cause an error, but the problem may not show up until these later
steps, so wrap those too.

Re #429.