Only run dsymutil on OSX in debug builds

When there are no debug symbols generated, then the program just prints an
annoying warning otherwise.

Closes #10198
Optimize creation of buffered readers/writers

I was benchmarking rust-http recently, and I saw that 50% of its time was spent
creating buffered readers/writers. Albeit rust-http wasn't using
std::rt::io::buffered, but the same idea applies here. It's much cheaper to
malloc a large region and not initialize it than to set it all to 0. Buffered
readers/writers never use uninitialized data, and their internal buffers are
encapsulated, so any usage of uninitialized slots are an implementation bug in
the readers/writers.
Removed test issue-912.rs

This was marked xfail-test
According to rust's issue #912, this will not be fixed.
There's no point in keeping the test if it is never intended to pass.
Remove run-pass/issue-3907-2.rs

This test is not supposed to compile; there is an equivalent test in compile-fail/issue-3907.rs.
Remove test issue-3461

According to issue #3461, it doesn't sound like this is planned to be fixed.
Remove resolve's xray infrastructure

Since the removal of privacy from resolve, this flag is no longer necessary to
get the test runner working. All of the privacy checks are bypassed by a special
item attribute in the privacy visitor.

Closes #4947
Update docs for Fractional, Algebraic, Round, and Trigonometric traits.
Add docs for traits Exponential, Hyperbolic, BitCount.
Got directions backwards in to_degrees and to_radians docs.
auto merge of #10424 : alexcrichton/rust/optimize-buffered, r=brson

I was benchmarking rust-http recently, and I saw that 50% of its time was spent
creating buffered readers/writers. Albeit rust-http wasn't using
std::rt::io::buffered, but the same idea applies here. It's much cheaper to
malloc a large region and not initialize it than to set it all to 0. Buffered
readers/writers never use uninitialized data, and their internal buffers are
encapsulated, so any usage of uninitialized slots are an implementation bug in
the readers/writers.
Use constants instead of raw values.
Move std::rt::io to std::io
auto merge of #10425 : astrieanna/rust/rm-issue-912, r=brson

According to rust's issue #912, this will not be fixed. There's no point in keeping the test if it is never intended to pass.
auto merge of #10198 : alexcrichton/rust/darwin-quiet, r=jdm

Instead of running dsymutil for all builds, this changes it to only run the program for debug builds.

Closes #3495
auto merge of #10435 : jayanderson/rust/docs, r=huonw

This fills in some missing docs in the nums package. Let me know if this is on the right track for what's wanted for docs. I can probably fill in more in the future. Thanks.

(As a side note the precedence of the unary negative operator '-' tripped me up for a bit. Essentially I would expect `-25.0f32.sqrt()` to result in NaN instead of `-5.0`.)
rt: Delete more C++
auto merge of #10439 : brson/rust/rmcxx, r=bstrie
Improve error message for breaks in blocks

Before it was always stated that it was a "break outside of a loop" when you
could very well be in a loop, but just in a block instead.

Closes #3064
auto merge of #10423 : alexcrichton/rust/move-io, r=pcwalton

I still plan on reorganizing the internals of the IO module a bit, but this is the major move which has been on the horizon for awhile.
auto merge of #10427 : alexcrichton/rust/no-xray, r=brson

Just a few triage issues that I ran into.