Define .narrow method to narrow numeric types

Can narrow either Rat or Num to Int, for instance.
Resolves #47.

Author: TimToady

S32-setting-library/Numeric.pod

@@ -19,8 +19,8 @@ DRAFT: Synopsis 32: Setting Library - Numeric
 
     Created: 19 Mar 2009 extracted from S29-functions.pod
 
-    Last Modified: 29 May 2013
-    Version: 17
+    Last Modified: 28 Dec 2013
+    Version: 18
 
 The document is a draft.
 
@@ -205,6 +205,15 @@ Convert from C<$base> to radians.
 
 Convert from radians to C<$base>.
 
+=item narrow
+
+ multi method narrow ( Numeric $x: ) is export
+
+Attempts to coerce the number to the narrowest type that can represent it accurately;
+for instance, a C<Rat> with a denominator of 1 maybe be coerced to an C<Int> instead;
+an integral C<Num> may likewise turn into an C<Int>.  (Neither C<Num> nor C<Rat> convert
+to each other, however.)
+
 =back