Use defvar for rust-top-item-beg-re instead

Since rust-top-item-beg-re hasn't been defined yet, using defvar instead of
setq is more appropriate here (and also silences compilation warnings).
Implement fallbacks for functions unavailable in older versions of Windows
Add documentation for Windows compatibility layer and remove unneeded attributes
Move Windows compatibility layer to libnative
shootout-nbody improvements

- minimize bound check
- factorise operations
- use x, y, z instead of [f64, ..3]
- ~1.15 faster
Reorganise driver code.

The goal of this refactoring is to make the rustc driver code easier to understand and use. Since this is as close to an API as we have, I think it is important that it is nice. On getting stuck in, I found that there wasn't as much to change as I'd hoped to make the stage... fns easier to use by tools.

This patch only moves code around - mostly just moving code to different files, but a few extracted method refactorings too. To summarise the changes: I added driver::config which handles everything about configuring the compiler. driver::session now just defines and builds session objects. I moved driver code from librustc/lib.rs to librustc/driver/mod.rs so all the code is one place. I extracted methods to make emulating the compiler without being the compiler a little easier. Within the driver directory, I moved code around to more logically fit in the modules.
add back jemalloc to the tree

This adds a `std::rt::heap` module with a nice allocator API. It's a
step towards fixing #13094 and is a starting point for working on a
generic allocator trait.

The revision used for the jemalloc submodule is the stable 3.6.0 release.

Closes #11807
add an align parameter to exchange_malloc

Closes #13094
use jemalloc to implement Vec<T>
initial port of the exchange allocator to jemalloc

In stage0, all allocations are 8-byte aligned. Passing a size and
alignment to free is not yet implemented everywhere (0 size and 8 align
are used as placeholders). Fixing this is part of #13994.

Closes #13616
fix Vec<ZeroSizeType>
vec: move some code inside alloc_or_realloc
vec: factor out some deallocation code
gitattributes: rm obsolete jemalloc entry
rename `global_heap` -> `libc_heap`

This module only contains wrappers for malloc and realloc with
out-of-memory checks.
rustdoc: Fix search links to enums/typedefs

When the values in html::item_type were updated, the JS definitions were
accidentally not updated as well.

Closes #14095
remove the word managed from the trait explanation
Clarification of Slice, Vector and Array

Especially in the tutorial beginners should not be confused with
wrong terminology. It helps to know the right names for things
when you want to find something in the documentation.
pass correct CFLAGS for jemalloc
android workaround