box: allow indexing of fixed-size numeric field types

The types added by commit 2e0f9a3c ("box: introduce fixed-size numeric
field types") can't be indexed. Fix it.

Perf results for `tuple_compare' benchmark with FIELD_TYPE_INTEGER:

NO_WRAP
$ taskset 0x2 ~/benchmark/tools/compare.py benchmarks \
  ./tuple.perftest.ref ./tuple.perftest.exp \
  --benchmark_min_warmup_time=10 \
  --benchmark_repetitions=30 \
  --benchmark_report_aggregates_only=true \
  --benchmark_filter=tuple_compare$
[...]
Comparing ./tuple.perftest.ref to ./tuple.perftest.exp
Benchmark                                    Time             CPU      Time Old      Time New       CPU Old       CPU New
-------------------------------------------------------------------------------------------------------------------------
tuple_tuple_compare_mean                  +0.0158         +0.0158            27            27            27            27
tuple_tuple_compare_median                +0.0153         +0.0152            27            27            27            27
tuple_tuple_compare_stddev                +0.0042         +0.0039             1             1             1             1
tuple_tuple_compare_cv                    -0.0114         -0.0117             0             0             0             0
NO_WRAP

Closes #9611

NO_DOC=bugfix
NO_CHANGELOG=unreleased