improve error messages
refactor to make combine less inherently commutative
further refactoring away from commutativity
Make tps invariant for now.  Fixes #1973.
continue refactoring to remove commutativity assumption
and introduce more indirection where it may be needed
rustc: Fail when there are multiple matches for 'use'
build: Remove a redundant search path
rustc: Warn when linking to multiple versions of the same crate

This is not something most people want to be doing and may
be a source of error.
test: Refactor the crateresolve tests

Keep their aux builds from stomping on each other
Convert old-style for loops to new-style

Most could use the each method, but because of the hack used to
disambiguate old- and new-style loops, some had to use vec::each.

(This hack will go away soon.)

Issue #1619
Remove support for old-style for

Closes #1619
Re-rename option functions

get_with_default (nee from_maybe) => get_default
with_option (nee maybe) => map_default
with_option_do (nee may) => iter

As per discussion of 21be1379d561b6679a8a2ea47dce88f948c5acca
adding uv::direct and beginning to work out tcp request case

lots of changes, here.. should've commited sooner.
- added uv::direct module that contains rust fns that map, neatly, to
the libuv c library as much as possible. they operate on ptrs to libuv
structs mapped in rust, as much as possible (there are some notable
exceptions). these uv::direct fns should only take inputs from rust and,
as neccesary, translate them into C-friendly types and then pass to the
C functions. We want to them to return ints, as the libuv functions do,
so we can start tracking status.
- the notable exceptions for structs above is due to ref gh-1402, which
prevents us from passing structs, by value, across the Rust<->C barrier
(they turn to garbage, pretty much). So in the cases where we get back
by-val structs from C (uv_buf_init(), uv_ip4_addr(), uv_err_t in callbacks)
, we're going to use *ctypes::void (or just errnum ints for uv_err_t) until
gh-1402 is resolved.
- using crust functions, in these uv::direct fns, for callbacks from libuv,
will eschew uv_err_t, if possible, in favor a struct int.. if at all
possible (probably isn't.. hm.. i know libuv wants to eventually move to
replace uv_err_t with an int, as well.. so hm).
- started flushing out a big, gnarly test case to exercise the tcp request
side of the uv::direct functions. I'm at the point where, after the
connection is established, we write to the stream... when the writing is
done, we will read from it, then tear the whole thing down.

overall, it turns out that doing "close to the metal" interaction with
c libraries is painful (and more chatty) when orchestrated from rust. My
understanding is that not much, at all, is written in this fashion in the
existant core/std codebase.. malloc'ing in C has been preferred, from what
I've gathered. So we're treading new ground, here!
some more stuff for libuv dealing w/ 1402.. should go away soon
changing ctypes:: to libc:: and impl of uv::direct::write()
impl of rustrt::rust_uv_write in c++ and whitespace cleanup
uv_write works, buffer passing still broke, can get sockaddr_in by val

.. but passing sockaddr_in by val back to C is broken, still passing by
ptr
.. the uv_write_cb is processed, but we have a status -1.. there is
also valgrind spew.. so buf passing is broken, still.
fixed passing in uv_buf_t ptr array in uv_write.. return status 0

ways to go, still..
fixed by-val from rust->c, use ++ sigil in native fn sig <-- NEVAR FORGET

have to use ++ sigil in rust-side extern fn decls in order to have rust
actually copy the struct, by value, onto the C stack. gotcha, indeed.

also adding a helper method to verify/remind how to pass a struct by-val
into C... check out the rust fn sig for rust_uv_ip4_test_verify_port_val()
for more infos
uv_buf_t's for uv_write() passed by-val .. no more mallocs or ptr cop-outs

so we're now adhering the libuv C api and passing structs by-val where
it is expected, instead of pulling pointer trickery (or worse having to
malloc structs in c++ to be passed back to rust and then into C again)