Move definition of ModuleMemoryOffset (#3228)

This was historically defined in `wasmtime-environ` but it's only used
in `wasmtime-cranelift`, so this commit moves the definition to the
`debug` module where it's primarily used.

crates/cranelift/src/debug.rs

@@ -2,6 +2,17 @@
 
 #![allow(clippy::cast_ptr_alignment)]
 
+/// Memory definition offset in the VMContext structure.
+#[derive(Debug, Clone)]
+pub enum ModuleMemoryOffset {
+    /// Not available.
+    None,
+    /// Offset to the defined memory.
+    Defined(u32),
+    /// Offset to the imported memory.
+    Imported(u32),
+}
+
 pub use write_debuginfo::{emit_dwarf, DwarfSection, DwarfSectionRelocTarget};
 
 mod gc;