Rust: Improve a few toString implementations

Author: paldepind

rust/ql/lib/codeql/rust/elements/internal/ImplTraitTypeReprImpl.qll

@@ -36,5 +36,9 @@ module Impl {
     predicate isInReturnPos() {
       exists(Function f | f.getRetType().getTypeRepr() = this.getFunctionTypeRepr(f))
     }
+
+    override string toStringImpl() {
+      result = "impl " + this.getTypeBoundList().toAbbreviatedString()
+    }
   }
 }

rust/ql/lib/codeql/rust/elements/internal/LifetimeImpl.qll

@@ -22,7 +22,9 @@ module Impl {
    * ```
    */
   class Lifetime extends Generated::Lifetime {
-    override string toStringImpl() {
+    override string toStringImpl() { result = this.toAbbreviatedString() }
+
+    override string toAbbreviatedString() {
       result = this.getText()
       or
       not this.hasText() and result = "'_"