add read-string and read-string! to (ice-9 rdelim)

* module/ice-9/rdelim.scm (read-string!, read-string): New functions.
* test-suite/tests/rdelim.test: Add tests.
* doc/ref/api-io.texi: Add docs.

* module/ice-9/iconv.scm:
* module/rnrs/io/ports.scm:
* module/web/uri.scm: Use the new functions.

doc/ref/api-io.texi

@@ -577,6 +577,33 @@ used.  This function is equivalent to:
 @end lisp
 @end deffn
 
+In the past, Guile did not have a procedure that would just read out all
+of the characters from a port.  As a workaround, many people just called
+@code{read-delimited} with no delimiters, knowing that would produce the
+behavior they wanted.  This prompted Guile developers to add some
+routines that would read all characters from a port.  So it is that
+@code{(ice-9 rdelim)} is also the home for procedures that can reading
+undelimited text:
+
+@deffn {Scheme Procedure} read-string [port] [count]
+Read all of the characters out of @var{port} and return them as a
+string.  If the @var{count} is present, treat it as a limit to the
+number of characters to read.
+
+By default, read from the current input port, with no size limit on the
+result.  This procedure always returns a string, even if no characters
+were read.
+@end deffn
+
+@deffn {Scheme Procedure} read-string! buf [port] [start] [end]
+Fill @var{buf} with characters read from @var{port}, defaulting to the
+current input port.  Return the number of characters read.
+
+If @var{start} or @var{end} are specified, store data only into the
+substring of @var{str} bounded by @var{start} and @var{end} (which
+default to the beginning and end of the string, respectively).
+@end deffn
+
 Some of the aforementioned I/O functions rely on the following C
 primitives.  These will mainly be of interest to people hacking Guile
 internals.