...(they still don't work).

Closes #5844 

Some machinery for enabling #[bench] benchmarks in std and some examples showing how to write them.

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.

This just redoes various parts of workcache to support context-cloning (eventually quite crudely, via ARCs), the absence of which was blocking rustpkg from being able to use it. Better versions of this are possible (notably removing the ARCs on everything except the database) but it ought to work well enough for now.

See #3994.

fixes #7689 

The following types are renamed:
```rust
block_ => Block
block => @mut Block

fn_ctx_ => FunctionContext
fn_ctx => @mut FunctionContext

scope_info => ScopeInfo
```
I also tried to convert instances of `@mut` to `&mut` or `&` but a lot of them are blocked by issue #6268, so I left it for some time later.

Factor out internal methods to pop/push list nodes so that .merge() and .rotate_to_front(), .rotate_to_back() (new methods) can be implemented without allocating nodes.

With that, some cleanup changes to DList use of Option, and adding a missing Encodable implementation.

SmallIntSet is equivalent to BitvSet but with 64 times the memory
overhead. There's no reason for it to exist.

SmallIntSet's overhead should really only be 8 times, but for some
reason, `sys::size_of::<Option<()>>() == 8`, not 1.

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.

Switched Bitv and BitvSet to external iterators. They still use some internal iterators internally (ha).

Derived clone for all Bitv types.

Removed indirection in BitvVariant. It previously held a unique pointer to the appropriate Bitv struct, even though those structs are the size of a pointer themselves. BitvVariant is the same size (16 bytes) as it was previously.

My first bit of newsched IO work. Pretty simple and limited in scope.

the RtioTimer trait only has a `sleep(msecs: u64)` method, for now. Taking requests on what else ought to be here.

oh yeah: this resolves #6435