Add 'lisp-toplevel' form to coalton syntax

lisp-toplevel may appear at any point in a coalton-toplevel or coalton
file context.

During code generation, lisp forms are merged with coalton definitions
according to the rule:

1. coalton forms are emitted in scc order
2. every time a coalton form is emitted, all lisp forms lexically preceding that form are emitted first

Convert math/num.lisp from interleaved lisp and coalton-toplevel forms
to a single coalton-toplevel form containing multiple lisp-toplevel forms.

Lisp-toplevel is, initially, restricted to library source code.

coalton.asd

@@ -27,7 +27,12 @@
   :version (:read-file-form "VERSION.txt")
   :around-compile (lambda (compile)
                     (let (#+sbcl (sb-ext:*derive-function-types* t)
-                          #+sbcl (sb-ext:*block-compile-default* :specified))
+                          #+sbcl (sb-ext:*block-compile-default* :specified)
+                          ;; The lisp-toplevel form is currently
+                          ;; restricted to standard library
+                          ;; implementation by checking for the
+                          ;; presence of this feature.
+                          (*features* (cons ':coalton-lisp-toplevel *features*)))
                       (funcall compile)))
   :defsystem-depends-on (#:coalton-asdf)
   :depends-on (#:coalton-compiler