Tweaks to srfi-41.scm.

* module/srfi/srfi-41.scm: Include relevant copyright notice for the
  copy of the code from srfi-45.scm.  Explain in more detail why it's
  there.
  (%stream-null): New variable.
  (stream-null, stream-null?): Use %stream-null.
  (stream-car, stream-cdr): Avoid double-forcing the promise.
  (stream-take): Check (zero? n) before checking (stream-null? strm),
  to avoid forcing any more than necessary.
Remove the unused and unexported 'pair-map'.

* module/srfi/srfi-41.scm (pair-map): Remove.
Make 'stream-fold-aux' an inlined procedure.

Suggested by Ian Price <ianprice90@googlemail.com>.

* module/srfi/srfi-41.scm (stream-fold-aux): Use 'define-inlinable'
  and change the optional 'limit' argument into a required argument.
  Move it above all uses.
  (stream-fold): Pass required 'limit' argument to 'stream-fold-aux'.
Add missing 'cond-expand-provide'.

* module/srfi/srfi-41.scm: Add missing 'cond-expand-provide'.
Use pass-if-equal instead of (pass-if (equal? ...)).

* test-suite/tests/srfi-41.test: Use pass-if-equal.
Minimal docs for SRFI-41.

* doc/ref/srfi-modules.texi (SRFI Support): Add SRFI-41.
  (SRFI-41): New node which refers the reader to
  <http://srfi.schemers.org/srfi-41/srfi-41.html>.
SRFI-41: Require that indices and counts be exact.

* module/srfi/srfi-41.scm (stream->list, stream-drop, stream-ref,
  stream-take): Require that the indices and counts be exact.
Add full documentation for SRFI-41.

* doc/ref/misc-modules.texi (Streams): Add cross-reference to SRFI-41.

* doc/ref/srfi-modules.texi (SRFI-41): Replace stub with full
  documentation.
  (SRFI-41 Stream Fundamentals, SRFI-41 Stream Primitives,
  SRFI-41 Stream Library): New subsubsections.
SRFI-41: Remove multiple-value support from embedded copy of SRFI-45.

* module/srfi/srfi-41.scm: Remove 2013 from list of FSF copyright dates
  on the embedded copy of SRFI-45.
  (stream-eager, stream-delay, stream-force): Revert changes that added
  multiple value-support to the embedded copy of SRFI-45.