[dylan] Add ilog2.

This is about to be added to Open Dylan, so add it to Mindy for
compatibility.

* libraries/dylan/transcendental.dylan
  (ilog2): Implement.

* libraries/dylan/library.dylan
  (module %Transcendental): Export ilog2.

* documentation/source/transcendentals.rst
  (log): Link to ilog2.
  (logn): Link to ilog2.
  (ilog2): Document.

documentation/source/transcendentals.rst

@@ -381,6 +381,7 @@ The following names are exported from the ``Transcendental`` module of the
    :seealso:
 
      - :gf:`exp`
+     - :func:`ilog2`
      - :gf:`logn`
 
 .. generic-function:: logn
@@ -402,6 +403,30 @@ The following names are exported from the ``Transcendental`` module of the
    :seealso:
 
      - :gf:`exp`
+     - :func:`ilog2`
+     - :gf:`log`
+
+.. function:: ilog2
+
+   :summary:
+     Returns the base two logarithm of its argument, truncated to an
+     integer.
+
+   :signature: ilog2 x => y
+
+   :parameter x: An instance of :drm:`<integer>`.
+   :value y: An instance of :drm:`<integer>`.
+
+   :description:
+
+     Returns the integer base two logarithm of ``x``, that is the
+     greatest integer less than or equal to the exact base two logarithm
+     of ``x``.
+
+   :seealso:
+
+     - :gf:`exp`
+     - :func:`logn`
      - :gf:`log`
 
 .. generic-function:: exp
@@ -422,6 +447,7 @@ The following names are exported from the ``Transcendental`` module of the
    :seealso:
 
      - :gf:`^`
+     - :func:`ilog2`
      - :gf:`log`
      - :gf:`logn`
 

libraries/dylan/library.dylan

@@ -424,4 +424,5 @@ define module %Transcendental
               exp, log, logn, sqrt, isqrt,
               $single-pi, $single-e, $double-pi, $double-e },
     export: all;
+  export ilog2;
 end module %Transcendental;

libraries/dylan/transcendental.dylan

@@ -51,5 +51,6 @@ define method \^ (num1 :: <double-float>, num2 :: <single-float>)
   num1 ^ as(<double-float>, num2);
 end method;
 
-
-
+define function ilog2 (x :: <integer>) => (result :: <integer>)
+  truncate(log(as(<single-float>, x)) / log(2.0s0))
+end function ilog2;