rustdoc: Fix file locking on windows

If the dwShareMode parameter is 0 on windows, it "prevents other processes from
opening a file or device if they request delete, read, or write access", which
is the opposite of what we want! This changes the 0 parameter to something which
will allow multiple processes to open the file and then lock it.
rustc: Switch defaults from libgreen to libnative

The compiler will no longer inject libgreen as the default runtime for rust
programs, this commit switches it over to libnative by default. Now that
libnative has baked for some time, it is ready enough to start getting more
serious usage as the default runtime for rustc generated binaries.

We've found that there isn't really a correct decision in choosing a 1:1 or M:N
runtime as a default for all applications, but it seems that a larger number of
programs today would work more reasonable with a native default rather than a
green default.

With this commit come a number of bugfixes:

* The main native task is now named "<main>"
* The main native task has the stack bounds set up properly
* #[no_uv] was renamed to #[no_start]
* The core-run-destroy test was rewritten for both libnative and libgreen and
  one of the tests was modified to be more robust.
* The process-detach test was locked to libgreen because it uses signal handling
rand: Rewrite OsRng in Rust for windows

This removes even more rust_builtin.c code, and allows us to more gracefully
handle errors (not a process panic, but a task failure).
rand: Fix a bug acquiring a context on windows

The details can be found in the comment I wrote on the block in question, but
the gist of it is that our usage of the TIB for a stack limit was causing
CryptAcquireContext to fail, so we temporarily get around it by setting the
stack limit to 0.
syntax: make OptVec immutable.

This is the first step to replacing OptVec with a new representation:
remove all mutability. Any mutations have to go via `Vec` and then make
to `OptVec`.

Many of the uses of OptVec are unnecessary now that Vec has no-alloc
emptiness (and have been converted to Vec): the only ones that really
need it are the AST and sty's (and so on) where there are a *lot* of
instances of them, and they're (mostly) immutable.
syntax: add the OwnedSlice vector wrapper.

This is a stand-in until we have a saner `~[T]` type (i.e. a proper
owned slice). It's a library version of what `~[T]` will be, i.e. an
owned pointer and a length.
Migrate all users of opt_vec to owned_slice, delete opt_vec.

syntax::opt_vec is now entirely unused, and so can go.
Optimized SipHash implementation
bench_compound_1: 70 -> 56
bench_long_str: 795 -> 525
bench_str: 32 -> 32