keysym-utf8: Optimise the keysym to utf8 lookup This change adds range checks based on the lowest keysym and highest keysym in the table. This allows a quick check to be applied to identify if the keysym is inside the table. To really give value to this optimisation the table is split to have a separate table for the keypad keysyms. The test suite passes with this change. Signed-off-by: Rob Bradford <rob@linux.intel.com>