Adds string context

Refs #114 and #1225. There are still many contexts to go, but these
are the three most important.

I'll also revise #1225 to close it when enough meaningful contexts
done. In general, most of them will be like

    * context

    Turns the object into * if possible, throws warning.

So there's not much to them...

Author: JJ

doc/Language/contexts.pod6

@@ -4,7 +4,7 @@
 
 =SUBTITLE What are contexts and how to get into them.
 
-A context is needed, in many occasions, to interpret the value of a container. In Perl 6, we will use context to coerce the value of a container into some type or class, or decide what to do with it, as in the case of tne sink context.
+A context is needed, in many occasions, to interpret the value of a container. In Perl 6, we will use context to coerce the value of a container into some type or class, or decide what to do with it, as in the case of the sink context.
 
 =head1 Sink
 
@@ -47,6 +47,18 @@ say True+$list; # OUTPUT: «4␤»
 
 In the case of I<listy> things, the numeric value will be in general equivalent to C<.elems>; in some cases, like L<Thread|/Numeric#(Thread)_method_Numeric> it will return an unique thread identifier.
 
+=head1 String
+
+In a I<string context>, values can be manipulated as strings. This context is used, for instance, for printing values to standard output.
+
+    say $very-complicated-and-hairy-object; # OUTPUT: something meaningful
+
+Or when smart-matching to a regular expression:
+
+    p6: say 333444777 ~~ /(3+)/; # OUTPUT: «｢333｣␤ 0 => ｢333｣␤»
+
+In general, the L<C<Str> routine|/routine/Str> will be called on a variable to contextualize it; since this method is inherited from L<Mu>, it is always present, but it is not always guaranteed to work. In some core classes it will issue a warning.
+
 =end pod
 
 # vim: expandtab softtabstop=4 shiftwidth=4 ft=perl6

xt/words.pws

@@ -1369,3 +1369,5 @@ rhs
 lhs
 RHS
 LHS
+contextualizers
+Procs