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.
Exposed previously hidden 'use' statements in the tutorial's sample code.
Note incompatibility with global-whitespace-mode.

See #3994.
Change libaux directory to fix aux crate tests on Windows
rt: Fix child-iteration bug in crate map.
std: add #[cfg(test)] reference to extra so we can benchmark libstd.
std: add preliminary str benchmark.
std: add #[bench] benchmarks for global and local heaps.
std: add #[bench] benchmarks for rand.
std: add #[bench] benchmarks for num::strconv
std: add benchmark for allocating-and-dropping a struct with a dtor.
std: various additional language benchmarks in util.
tutorial: Repair broken links
tutorial: Fix obsolete names
Add link_section attribute for static and fn items

This allows for control over the section placement of static, static
mut, and fn items.  One caveat is that if a static and a static mut are
placed in the same section, the static is declared first, and the static
mut is assigned to, the generated program crashes.  For example:

#[link_section=".boot"]
static foo : uint = 0xdeadbeef;

#[link_section=".boot"]
static mut bar : uint = 0xcafebabe;

Declaring bar first would mark .bootdata as writable, preventing the
crash when bar is written to.
tutorial: Remove the sentence about mutable fields.
Add a test for #5844 (a closed issue now)
test/run-fail: Un-xfail a test, make some other xfailed ones compile (they still don't work).