Add dependence arc from running rustc to its libraries.

This commit fixes some oversights in the Makefile where rustc could be
invoked without some of its dependencies yet in place.  (I encountered
the problem in practice; its not just theoretical.)

As written in Makefile.in, $(STAGE$(1)_T_$(2)_H_$(3)) is the way one
writes an invocation of rustc where $(1) is the stage number $(2) is
the target triple $(3) is the host triple.  (Other uses of the macro
may plug in actual values or different parameters in for those three
formal parameters.)

When you have invocations of $(STAGE...), you need to make sure that
its dependences are satisfied; otherwise, if someone is using `make
-jN` for certain (large-ish) `N`, one can encounter situations where
GNU make attempts to invoke `rustc` before it has actually copied some
of its libraries into place, such as libmorestack.a, which causes a
link failure when the rustc invocation attempts to link in those
libraries.

In this case, the main prerequisite to add is TSREQ$(1)_T_$(2)_H_$(3),
which is described in Makefile.in as "Prerequisites for using the
stageN compiler to build target artifacts"

----

In addition to adding the extra dependences on TSREQ..., I also
replaced occurrences of the pattern:

    TSREQ$(1)_T_$(2)_H_$(3)
    $$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_STDLIB_$(2))
    $$(TLIB$(1)_T_$(2)_H_$(3))/$(CFG_EXTRALIB_$(2))

with:

    SREQ$(1)_T_$(2)_H_$(3)

which is equivalent to the above, as defined in Makefile.in

----

Finally, for the cases where TSREQ was missing in tests.mk, I went
ahead and put in a dependence on SREQ rather than just TSREQ, since it
was not clear to me how one could expect to compile those cases
without stdlib and extralib.

(It could well be that I should have gone ahead and done the same in
 other cases where I saw TSREQ was missing, and put SREQ in those
 cases as well.  But this seemed like a good measure for now, without
 needing to tax my understanding of the overall makefile
 infrastructure much further.)
option: Title-case `Some` and `None` in docs and fail messages

For accuracy, say 'get_ref None' instead of 'get_ref none', and so on.
option: Add .chain_mut_ref()

.chain_mut_ref() is the mutable alternative to .chain_ref().

A use case example for this method is extraction of an optional value
from an Option<Container> value.
serialize: implement Encodable for DList

This impl was missing for unknown reason.
dlist: Simplify match clauses to use Option methods

Make the core Deque implementation a bit simpler by using Option methods
when we simply map on a Some value, and deduplicate some common lines.
dlist: Factor out pop and push operations by list node

Factor out internal methods for pop/push ~Node<T>, This allows moving
nodes instead of destructuring and allocating new.

Make use of this in .merge() so that it requires no allocations when
merging two DList.
dlist: Add .rotate_to_front(), .rotate_to_back()

Add methods to move back element to front or front element to back,
without reallocating nodes.
dlist: Add bench test for rotate_to_{front, back}
dlist: Remove bench tests for vec

These tests for ~[] performance don't really belong here, they were for
comparison.
dlist: Use Ord for .insert_ordered()

We don't need TotalOrd for ordered insertion, just the normal sort order
Ord.
dlist: Remove extraneous unwrap in .pop_back_node()
Eliminated SmallIntSet

SmallIntSet is equivalent to BitvSet but with 8 times the memory
overhead. There's no reason for it to exist.
Minor cleanup of hashmap
Exposed previously hidden 'use' statements in the tutorial's sample code.
Note incompatibility with global-whitespace-mode.

See #3994.
dlist: Fix .peek_next() w.r.t double ended iterators

.peek_next() needs to check the element counter just like the .next()
and .next_back() iterators do.

Also clarify .insert_next() doc w.r.t double ended iteration.
Change libaux directory to fix aux crate tests on Windows
dlist: Rename rotate methods to .rotate_forward() and .rotate_backward()
rt: Fix child-iteration bug in crate map.
std: add #[cfg(test)] reference to extra so we can benchmark libstd.