Add generic perfect string hashes, use for directives

Add generic perfect string hashes, use for directives

Add a generic facility for generating perfect string hashes, where all
that is needed is an enum and a string table.  The existing mechanism
using a custom Perl script wrapped around a module continues to be
available for any use case where this particular approach isn't
sophisticated enough.

Much of this patch comes from renaming "enum directives" to "enum
directive" as a result of the string hash generator expecting a set of
uniform naming conventions.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>