Propagate optional import names to the wasmtime/C API

With the module linking proposal the field name on imports is now
optional, and only the module is required to be specified. This commit
propagates this API change to the boundary of wasmtime's API, ensuring
consumers are aware of what's optional with module linking and what
isn't. Note that it's expected that all existing users will either
update accordingly or unwrap the result since module linking is
presumably disabled.

crates/environ/src/module.rs

@@ -163,7 +163,7 @@ pub struct Module {
     pub name: Option<String>,
 
     /// All import records, in the order they are declared in the module.
-    pub imports: Vec<(String, String, EntityIndex)>,
+    pub imports: Vec<(String, Option<String>, EntityIndex)>,
 
     /// Exported entities.
     pub exports: IndexMap<String, EntityIndex>,