tamer: tameld: Reduce peak memory usage

This clears the buffers used by quick_xml, which was apparently forgotten
during initial development (I think I expected it to re-use the previously
allocated space automatically).

This has significant effects in some cases.  For example, one of our UI
builds drops from ~9KiB to ~5KiB peak memory usage.  Other builds for larger
suppliers are only slightly effected because of some of their massive
fragments.

RELEASES.md

@@ -24,6 +24,11 @@ Linker
 ------
 - Remove exception for input map dependency processing (now that compiler no
   longer emits such a dependency).
+- Reduce peak memory usage by clearing buffer of `xmlo` reader between
+  events.
+  - How effective this is varies depending on the size of individual
+    entities within the XML document.  In some cases, it can reduce peak
+    memory usage by half.
 
 
 v18.0.3 (2021-07-21)

tamer/src/obj/xmlo/reader/mod.rs

@@ -250,6 +250,11 @@ where
     ///
     /// TODO: Augment failures with context
     pub fn read_event<'a>(&mut self) -> XmloResult<XmloEvent<Ix>> {
+        // Just to cut down on peak memory usage, cleaning up after a
+        // previous run.  This does not affect behavior.
+        self.buffer.clear();
+        self.sub_buffer.clear();
+
         let event = self.reader.read_event(&mut self.buffer)?;
 
         // Ensure that the first encountered node is something we expect