This allocator has no purpose since it cannot truly fulfill the role of
page allocation, and std.heap.wasm_allocator is better both in terms of
performance and code size.
This commit redefines `std.heap.page_allocator` to be less strict:
"On operating systems that support memory mapping, this allocator makes
a syscall directly for every allocation and free. Otherwise, it falls
back to the preferred singleton for the target. Thread-safe."
This now matches how it was actually being implemented, and matches its
use sites - which are mainly as the backing allocator for
`std.heap.ArenaAllocator`.
fecdc53a48