Refactor and extract call_reducer and update_database machinery for reuse by V8 (#3190)

# Description of Changes

Refactors the machinery of call_reducer and update_database to
a) have smaller pieces that make up the whole so that the code becomes
clearer
b) extract all the VM-independent stuff so that it can be reused by V8
modules.

This is best reviewed commit by commit.

# API and ABI breaking changes

None.

# Expected complexity level and risk

2, it's an important place, but this is doing just code motion.

# Testing

No semantic changes, just code motion.

Author: Centril

crates/core/src/host/v8/key_cache.rs

@@ -22,7 +22,7 @@ pub(super) struct KeyCache {
     tag: Option<Global<v8::String>>,
     /// The `value` property for sum values in JS.
     value: Option<Global<v8::String>>,
-    /// The `describe_module` property on the global proxy object.
+    /// The `__describe_module__` property on the global proxy object.
     describe_module: Option<Global<v8::String>>,
 }
 
@@ -37,9 +37,9 @@ impl KeyCache {
         Self::get_or_create_key(scope, &mut self.value, "value")
     }
 
-    /// Returns the `describe_module` property name.
+    /// Returns the `__describe_module__` property name.
     pub(super) fn describe_module<'scope>(&mut self, scope: &mut HandleScope<'scope>) -> Local<'scope, v8::String> {
-        Self::get_or_create_key(scope, &mut self.describe_module, "describe_module")
+        Self::get_or_create_key(scope, &mut self.describe_module, "__describe_module__")
     }
 
     /// Returns an interned string corresponding to `string`

crates/core/src/host/v8/mod.rs

@@ -134,9 +134,9 @@ impl ModuleInstance for JsInstance {
     }
 }
 
-// Calls the `describe_module` function on the global proxy object to extract a [`RawModuleDef`].
+// Calls the `__describe_module__` function on the global proxy object to extract a [`RawModuleDef`].
 fn call_describe_module(scope: &mut HandleScope<'_>) -> anyhow::Result<RawModuleDef> {
-    // Get a cached version of the `describe_module` property.
+    // Get a cached version of the `__describe_module__` property.
     let key_cache = get_or_create_key_cache(scope);
     let describe_module_key = key_cache.borrow_mut().describe_module(scope).into();
 
@@ -150,7 +150,7 @@ fn call_describe_module(scope: &mut HandleScope<'_>) -> anyhow::Result<RawModule
 
         // Convert to a function.
         let fun =
-            cast!(scope, object, Function, "function export for `describe_module`").map_err(|e| e.throw(scope))?;
+            cast!(scope, object, Function, "function export for `__describe_module__`").map_err(|e| e.throw(scope))?;
 
         // Call the function.
         let receiver = v8::undefined(scope).into();
@@ -183,7 +183,7 @@ mod test {
     #[test]
     fn call_describe_module_works() {
         let code = r#"
-            function describe_module() {
+            function __describe_module__() {
                 return {
                     "tag": "V9",
                     "value": {