xkbcomp/scanner: avoid unneeded strdup of IDENT tokens

[bluetech]

The allocation is immediately discarded, either turned into a keysym or
an atom. So use an sval slice into the input string instead strdup'ing.

memusage ./release/bench-compile-keymap --iter=1000 --layout us,de --variant ,neo

Before:

Memory usage summary: heap total: 534063576, heap peak: 581022, stack peak: 18848
        total calls   total memory   failed calls
 malloc|   11240525      291897104              0
realloc|    1447657      192307328              0  (nomove:37629, dec:0, free:0)
 calloc|     430573       49859144              0
   free|   13993903      534063576

After:

Memory usage summary: heap total: 506839909, heap peak: 581022, stack peak: 18960
        total calls   total memory   failed calls
 malloc|    8016419      264673437              0
realloc|    1447657      192307328              0  (nomove:37278, dec:0, free:0)
 calloc|     430573       49859144              0
   free|    0769797      506839909