GN_Lut, Lookup table facility

IN: BHL = lookup table address (B=0, local)
    CDE = key (null terminated string) to search
    A = search options
        A0 = 1, case sensitive

        A0 = 0, case insensitive

 Lookup table format : keys have to be alphabetically sorted (ASCII sorted table if symbol are used)
 ---------------------

 defb N                       ; number of entries
 defm "key1",0, pointer1      ; first entry, null terminated followed by a 16bit pointer
 defm "key2",0, pointer2      ; second entry
 ...
 defm "keyN",0, pointerN      ; Nth and last entry

out, if successful:

        Fc = 0, key found and associated pointer returned in HL

out, if failed:

        Fc = 1, A = error code

             RC_Eof, no matching string found

   registers changed after return:

        A.BC.E../IXIY  same

        .F..D.HL/....  different