Fix a memory reservation bug in reserve_modules

This method attempted to reserve space in the `results` list of final
modules. Unfortunately `results.reserve(nmodules)` isn't enough here
because this can be called many times before a module is actually
finished and pushed onto the vector. The attempted logic to work around
this was buggy, however, and would simply trigger geometric growth on
every single reservation because it erroneously assumed that a
reservation would be exactly met.

This is fixed by avoiding looking at the vector's capacity and instead
keeping track of modules-to-be in a side field. This is the incremented
and passed to `reserve` as it represents the number of modules that will
eventually make their way into the result vector.

crates/environ/src/module_environ.rs

@@ -31,6 +31,10 @@ pub struct ModuleEnvironment<'data> {
     /// the module linking proposal.
     results: Vec<ModuleTranslation<'data>>,
 
+    /// How many modules that have not yet made their way into `results` which
+    /// are coming at some point.
+    modules_to_be: usize,
+
     /// Intern'd types for this entire translation, shared by all modules.
     types: TypeTables,
 
@@ -138,6 +142,7 @@ impl<'data> ModuleEnvironment<'data> {
         Self {
             result: ModuleTranslation::default(),
             results: Vec::with_capacity(1),
+            modules_to_be: 1,
             cur: 0,
             types: Default::default(),
             target_config,
@@ -741,8 +746,8 @@ and for re-adding support for interface types you can see this issue:
     fn reserve_modules(&mut self, amount: u32) {
         // Go ahead and reserve space in the final `results` array for `amount`
         // more modules.
-        let extra = self.results.capacity() + (amount as usize) - self.results.len();
-        self.results.reserve(extra);
+        self.modules_to_be += amount as usize;
+        self.results.reserve(self.modules_to_be);
 
         // Then also reserve space in our own local module's metadata fields
         // we'll be adding to.
@@ -796,6 +801,7 @@ and for re-adding support for interface types you can see this issue:
         self.cur = index;
         assert_eq!(index, self.results.len());
         self.results.push(prev);
+        self.modules_to_be -= 1;
     }
 
     fn module_end(&mut self, index: usize) {