move most of runtime/vm/cow.rs over to aligned byte counts (#9652)

* move most of runtime/vm/cow.rs over to aligned byte counts

As part of attempting to move some of these operations over to Mmap instances,
it is nice to have type-level checking for aligned sizes. In upcoming PRs, APIs
like `map_at` will be switched to using `Mmap` instances with aligned counts.

There are a couple of spots where I have questions -- will flag them in review
comments.

* address review comments, incl workaround for #9660

crates/wasmtime/src/runtime/vm.rs

@@ -367,14 +367,6 @@ pub fn host_page_size() -> usize {
     };
 }
 
-/// Is `bytes` a multiple of the host page size?
-///
-/// (Deprecated: consider switching to `HostAlignedByteCount`.)
-#[cfg(feature = "signals-based-traps")]
-pub fn usize_is_multiple_of_host_page_size(bytes: usize) -> bool {
-    bytes % host_page_size() == 0
-}
-
 /// Round the given byte size up to a multiple of the host OS page size.
 ///
 /// Returns an error if rounding up overflows.