keysyms: Pad Unicode keysyms only up to 4 digits Previously there was a distinction between keysyms with code points in BMP and the others: the former used a 4-padding while the latter used a 8-padding: e.g `U0001` vs `U00010000`. This is unnecessary and makes the reading harder. Let’s use the same padding for all: `U0001` and `U10000`. Parsing remains unchanged and would parse both paddings. Also added a test to check no explicit name can clash with Unicode notation.