`std.GeneralPurposeAllocator` is now available. It is a function that
takes a configuration struct (with default field values) and returns an
allocator. There is a detailed description of this allocator in the
doc comments at the top of the new file.

The main feature of this allocator is that it is *safe*. It
prevents double-free, use-after-free, and detects leaks.

Some deprecation compile errors are removed.

The Allocator interface gains `old_align` as a new parameter to
`resizeFn`. This is useful to quickly look up allocations.

`std.heap.page_allocator` is improved to use mmap address hints to avoid
obtaining the same virtual address pages when unmapping and mapping
pages. The new general purpose allocator uses the page allocator as its
backing allocator by default.

`std.testing.allocator` is replaced with usage of this new allocator,
which does leak checking, and so the LeakCheckAllocator is retired.

stage1 is improved so that the `@typeInfo` of a pointer has a lazy value
for the alignment of the child type, to avoid false dependency loops
when dealing with pointers to async function frames.

The `std.mem.Allocator` interface is refactored to be in its own file.

`std.Mutex` now exposes the dummy mutex with `std.Mutex.Dummy`.

This allocator is great for debug mode, however it needs some work to
have better performance in release modes. The next step will be setting
up a series of tests in ziglang/gotta-go-fast and then making
improvements to the implementation.

 * std.Mutex API is improved to not have init() deinit(). This API is
   designed to support static initialization and does not require any
   resource cleanup. This also happens to work around some kind of
   stage1 behavior that wasn't letting the new allocator mutex code
   get compiled.
 * the general purpose allocator now returns a bool from deinit()
   which tells if there were any leaks. This value is used by the test
   runner to fail the tests if there are any.
 * self-hosted compiler is updated to use the general purpose allocator
   when not linking against libc.

This makes `@returnAddress()` return 0 for WebAssembly (when not using
the Emscripten OS) and avoids trying to capture stack traces for the
general purpose allocator on that target.

even outside of unit tests

Sadly, trying to collect stack frames goes into an infinite loop on
mips. This sets the default number of stack frames to collect to 0 on
mips.

We don't pass no-omit-frame-pointer in release safe by default, so it
also makes sense to not try to collect stack trace frames by default in
release safe mode.

The tests are cleverly testing some alignment stuff, but were getting
thwarted by Windows choosing to allocate 64K aligned pages.