Revert removal of unreadCh change for performance

Turns out unreadCh is much more efficient than seek(-1, SEEK_CUR).
Update comments and code to reflect this.

1 files changed, 0 insertions, 4 deletions

diff --git a/TODO b/TODO
index 80dbb9a9..29bb38ab 100644
--- a/TODO
+++ b/TODO
@@ -157,10 +157,6 @@ ABI Changes
 This is a list of changes to make next time there is an ABI change.
 Comments appear in the code prefixed by "ABI"
 
-* Consider removing InputSource::unreadCh. Maybe we can declare it
-  final and delete so it will be forced to be removed from derived
-  classes.
-
 C++-11
 ======