[S15] Change .chr(s) to return Uni, not Str.

This is because Uni is the only string type that won't convert strings
given to it, which for people who are playing with numeric arrays of
codepoints is probably important.

Author: ShimmerFairy

S15-unicode.pod

@@ -303,11 +303,11 @@ type C<Str> may return invalid negative numbers as "codepoints".
 
 =head2 Character Representation
 
-    chr(Int $codepoint) --> Str
-    chrs(Array[Int] @codepoints) --> Str
+    chr(Int $codepoint) --> Uni
+    chrs(Array[Int] @codepoints) --> Uni
 
-    Cool.chr() --> Str
-    Cool.chrs() --> Str
+    Cool.chr() --> Uni
+    Cool.chrs() --> Uni
 
 Converts one or more numbers into a series of characters, treating those numbers
 as Unicode codepoints. The C<chrs> version generates a multi-character string
@@ -316,13 +316,12 @@ from the given array.
 Note that this operates on encoding-independent codepoints (use C<Buf> types for
 encoded codepoints).
 
-An error will occur if the C<Str> generated by these functions contains an
+An error will occur if the C<Uni> generated by these functions contains an
 invalid character or sequence of characters. This includes, but is not limited
-to, codepoint values greater than C<0x10FFFF> and parts of surrogate code pairs,
-and negative numbers.
+to, codepoint values greater than C<0x10FFFF> and parts of surrogate code pairs.
 
-The ability to convert negative number codepoints (i.e. C<Str.ords>) is not
-guaranteed and therefore currently disallowed.
+To obtain a more definitive string type, the normal ways of type conversion may
+be used.
 
 =head2 Character Name