Fix typeahead

[gwenn]

Fix #690

To test, you need to activate the feature like this:

diff --git a/Cargo.toml b/Cargo.toml
index 7ac2645..fd27bb7 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -87,7 +87,7 @@ name = "input_validation"
 required-features = ["derive"]
 [[example]]
 name = "numeric_input"
-required-features = ["custom-bindings"]
+required-features = ["custom-bindings", "buffer-redux"]
 [[example]]
 name = "read_password"
 required-features = ["derive"]