Use mktemp for temporary download directory

Using the current directory may not always be appropriate, for example in
the case where it will unnecessarily trigger a backup to be made.

The only risk with this change is that systems might not have a mktemp.
I am not aware of such a system, but have not tested on Windows. It is
working on a basic Ubuntu and OS X installation.
Fall back to hard coded download directory

If mktemp fails, fall back to a hard coded directory, per @nodakai's feedback.
Several fixes for DragonFly (rebase)
Detect type inference failure when auto-dereferencing a pointer

check::autoderef() returns a ty_err when it fails to infer the type.
probe::probe() should respect this failure and fail together to prevent
further corruption.

Call stack: check::check_method_call() -> method::lookup() ->
            probe::probe() + confirm::confirm()

Fixes #19692.
Fixes #19583.
Fixes #19297.
load_self() needs to be public
Implement `reserve_len` and `reserve_len_exact` for VecMap
Use type-alignment-sized integer for discriminant types

The previous behaviour of using the smallest type possible caused LLVM
to treat padding too conservatively, causing poor codegen. This commit
changes the behaviour to use an type-alignment-sized integer as the
discriminant. This keeps types the same size, but helps LLVM understand
the data structure a little better, resulting in better codegen.
Add enum size cases to type-sizes test
Make alignment-forcing case work the same on 64 and 32-bit platforms
Rename include_bin! to include_bytes!

According to [RFC 344][], methods that return `&[u8]` should have names
ending in `bytes`. Though `include_bin!` is a macro not a method, it
seems reasonable to follow the convention anyway.

We keep the old name around for now, but trigger a deprecation warning
when it is used.

[RFC 344]: https://github.com/rust-lang/rfcs/blob/master/text/0344-conventions-galore.md

[breaking-change]
debuginfo: Create debuginfo for for-loop variables again.
debuginfo: Clean the debuginfo module up a bit.
debuginfo: Add test case for destructured for-loop variable.
Add structs for ranges to core::ops.
Add lang items for ranges.
Add syntax for ranges
Type checking and trans for ranges
Tests
Review changes
Support all variants of WherePredicate

Adds support for all variants of ast::WherePredicate in clean/mod.rs. Fixes #20048, but will need modification when EqualityPredicates are fully implemented in #20041.