Refactor C FFI variadics to more closely match their C counterparts, and add Clone implementation
We had to make some changes to expose `va_copy` and `va_end` directly to users (mainly for C2Rust, but not exclusively):
- redefine the Rust variadic structures to more closely correspond to C: `VaList` now matches `va_list`, and `VaListImpl` matches `__va_list_tag`
- add `Clone` for `VaListImpl`
- add explicit `as_va_list()` conversion function from `VaListImpl` to `VaList`
- add deref coercion from `VaList` to `VaListImpl`
- add support for the `asmjs` target
All these changes were needed for use cases like:
```Rust
let mut ap2 = va_copy(ap);
vprintf(fmt, ap2);
va_end(&mut ap2);
```
Limit dylib symbols
This makes `windows-gnu` match the behavior of `windows-msvc`. It probably doesn't make sense to export these symbols on other platforms either.
Make visitors iterate
r? @oli-obk
The second commit is not completely equivalent, unsure if the code is wrong or not. Tests pass though, otherwise we would need to iterate in the opposite direction as it happened in other parts of the code.
add support for unchecked math
add compiler support for
```rust
/// Returns the result of an unchecked addition, resulting in
/// undefined behavior when `x + y > T::max_value()` or `x + y < T::min_value()`.
pub fn unchecked_add<T>(x: T, y: T) -> T;
/// Returns the result of an unchecked substraction, resulting in
/// undefined behavior when `x - y > T::max_value()` or `x - y < T::min_value()`.
pub fn unchecked_sub<T>(x: T, y: T) -> T;
/// Returns the result of an unchecked multiplication, resulting in
/// undefined behavior when `x * y > T::max_value()` or `x * y < T::min_value()`.
pub fn unchecked_mul<T>(x: T, y: T) -> T;
```
cc https://github.com/rust-lang/rfcs/issues/2508
light refactoring of global AllocMap
* rename AllocKind -> GlobalAlloc. This stores the allocation itself, not just its kind.
* rename the methods that allocate stuff to have consistent names.
Cc @oli-obk
Make the `type_name` intrinsic deterministic
cc @eddyb for the printing infrastructure
cc @Centril for the deterministic (coherent?) output
r? @sfackler
Avoid `hygiene_data` lookups
These commits mostly introduce compound operations that allow two close adjacent `hygiene_data` lookups to be combined.
r? @petrochenkov
* rename AllocKind -> GlobalAlloc. This stores the allocation itself, not just its kind.
* rename the methods that allocate stuff to have consistent names.
