Declarations not supported by x-let varients

[AkashaP]

A problem i found whilst scouring Alexandria code. it causes this to fail:

(using either alexandria or if-letstar at commit #63c5cef797c887e7db9391be639ec19e35f4d69e or before)

(if-let* ((a :foo) (b :bar))
      (declare (type symbol a b))
      t
      nil)

fails at runtime because it tries to execute (declare (type symbol a b)) as the true path, and at compile-time because for example you can't put declare inside a progn block (common in macros) it has to be right after the bindings and such.

more examples of code that cant be done in various x-let varients when it seems reasonable the syntax can fit it otherwise:

(labels ((c () (print a) (print b)))
      (if-let* ((a :foo) (b (eq 1 1)))
        (declare (special a))
        (declare (special b))
        (declare (type symbol a b))
        (progn
          (print "true")
          (c))
        (progn
          (print "false")
          (c))))