Refactor shared memory internals, expose embedder methods (#5311)

This commit refactors the internals of `wasmtime_runtime::SharedMemory`
a bit to expose the necessary functions to invoke from the
`wasmtime::SharedMemory` layer. Notably some items are moved out of the
`RwLock` from prior, such as the type and the `VMMemoryDefinition`.
Additionally the organization around the `atomic_*` methods has been
redone to ensure that the `wasmtime`-layer abstraction has a single
method to call into which everything else uses as well.

crates/runtime/src/vmcontext.rs

@@ -12,7 +12,7 @@ use std::ptr::NonNull;
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::u32;
 pub use vm_host_func_context::VMHostFuncContext;
-use wasmtime_environ::{DefinedMemoryIndex, Trap};
+use wasmtime_environ::DefinedMemoryIndex;
 
 pub const VMCONTEXT_MAGIC: u32 = u32::from_le_bytes(*b"core");
 
@@ -248,30 +248,6 @@ impl VMMemoryDefinition {
             current_length: other.current_length().into(),
         }
     }
-
-    /// In the configurations where bounds checks were elided in JIT code (because
-    /// we are using static memories with virtual memory guard pages) this manual
-    /// check is here so we don't segfault from Rust. For other configurations,
-    /// these checks are required anyways.
-    pub fn validate_addr(
-        &self,
-        addr: u64,
-        access_size: u64,
-        access_alignment: u64,
-    ) -> Result<*const u8, Trap> {
-        debug_assert!(access_alignment.is_power_of_two());
-        if !(addr % access_alignment == 0) {
-            return Err(Trap::HeapMisaligned);
-        }
-
-        let length = u64::try_from(self.current_length()).unwrap();
-        if !(addr.saturating_add(access_size) < length) {
-            return Err(Trap::MemoryOutOfBounds);
-        }
-
-        // SAFETY: checked above that the address is in bounds
-        Ok(unsafe { self.base.add(addr as usize) })
-    }
 }
 
 #[cfg(test)]