This functionality is used in multiple places in Qt itself, so it makes
sense to have a global function for this. This also allows to map this
onto specialized assembler instructions, should an architecture provide
them, later on.
Also added comprehensive tests, using a 4-bit lookup-table implementation
as a reference.
Change-Id: I8c4ea72cce54506ebb9fbe61141dbb5f1b7a660f
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>