Remove dependency on the region crate (#4407)

This commit removes Wasmtime's dependency on the `region` crate. The
motivation for this came about when I was updating dependencies and saw
that `region` had a new major version at 3.0.0 as opposed to our
currently used 2.3 track. In reviewing the use cases of `region` within
Wasmtime I found two trends in particular which motivated this commit:

* Some unix-specific areas of `wasmtime_runtime` use
  `rustix::mm::mprotect` instead of `region::protect` already. This
  means that the usage of `region::protect` for changing virtual memory
  protections was already inconsistent.

* Many uses of `region::protect` were already in unix-specific regions
  which could make use of `rustix`.

Overall I opted to remove the dependency on the `region` crate to avoid
chasing its versions over time. Unix-specific changes of protections
were easily changed to `rustix::mm::mprotect`. There were two locations
where a windows/unix split is now required and I subjectively ruled
"that seems ok". Finally removing `region` also meant that the "what is
the current page size" query needed to be inlined into
`wasmtime_runtime`, which I have also subjectively ruled "that seems
fine".

Finally one final refactoring here was that the `unix.rs` and `linux.rs`
split for the pooling allocator was merged. These two files already only
differed in one function so I slapped a `cfg_if!` in there to help
reduce the duplication.

crates/runtime/src/cow.rs

@@ -212,7 +212,7 @@ impl ModuleMemoryImages {
             _ => return Ok(None),
         };
         let mut memories = PrimaryMap::with_capacity(map.len());
-        let page_size = region::page::size() as u32;
+        let page_size = crate::page_size() as u32;
         for (memory_index, init) in map {
             // mmap-based-initialization only works for defined memories with a
             // known starting point of all zeros, so bail out if the mmeory is
@@ -596,7 +596,7 @@ mod test {
 
     fn create_memfd_with_data(offset: usize, data: &[u8]) -> Result<MemoryImage> {
         // Offset must be page-aligned.
-        let page_size = region::page::size();
+        let page_size = crate::page_size();
         assert_eq!(offset & (page_size - 1), 0);
         let memfd = create_memfd()?;
         memfd.as_file().write_all(data)?;