`b.Loop()` drastically shortens benchmark execution times for tests with
an expensive initialization phase as it only has to happen once now.

…blobs

Only in use on 64-bit systems. Use the upper 28bits of the id of an
index entry as bloom filter. This allows skipping the index entry
traversal most of the time if an id is not stored in the hashmap.

The bloom filter embedded in the index entry id is check each time
before following a reference to an index entry. This further reduces
the risk of false positives. The bloom filter itself is basically for
free on modern CPUs.

The main performance cost of checking for unknown blobs in the index are
the essentially random RAM accesses for the initial bucket lookup as
well as following the next pointer in the index entries. With the bloom
filter most of the time only the initial bucket lookup is necessary.

This speeds up checking for unknown blobs by a factor 5 (!), while
having no effect on the lookup of known blobs:

$ benchstat no-bloom with-bloom
name                old time/op  new time/op  delta
IndexHasUnknown-16  49.0ms ± 2%   9.9ms ± 7%  -79.70%  (p=0.000 n=10+10)
IndexHasKnown-16    48.0ms ± 3%  47.9ms ± 3%     ~     (p=0.968 n=10+9)

This bloom filter parameters m=28 k=1 were derived empirically, while
also leaving sufficient room for very large repositories. Before this
commit, the final merge index step took roughly 1 second per million
index entries. With the chosen bloom filter parameters, it would
currently take 19 hours to just merge such an index. It is safe to
assume that such large repositories don't exist.

Comparison with other parameter sets:

$ m=28 k=1 versus m=32 k=1
name                old time/op  new time/op  delta
IndexHasUnknown-16  49.0ms ± 2%   9.7ms ±16%  -80.17%  (p=0.000 n=10+10)
IndexHasKnown-16    48.0ms ± 3%  48.4ms ± 3%     ~     (p=0.436 n=10+10)

$ m=28 k=1 versus m=24 k=1
name                old time/op  new time/op  delta
IndexHasUnknown-16  49.0ms ± 2%  10.8ms ±13%  -77.90%  (p=0.000 n=10+10)
IndexHasKnown-16    48.0ms ± 3%  47.9ms ± 3%     ~     (p=0.684 n=10+10)

$ m=28 k=1 versus m=28 k=2
name                old time/op  new time/op  delta
IndexHasUnknown-16  49.0ms ± 2%  24.9ms ± 5%  -49.27%  (p=0.000 n=10+10)
IndexHasKnown-16    48.0ms ± 3%  48.0ms ± 4%     ~     (p=1.000 n=10+10)

`k=2` outright wrecks the performance. This is most likely the case as
it performs worse on longer index entry chains, which also happen to be
the expensive ones to process.

`m=32` yields diminishing returns, while getting within an order of
magnitude of the largest known restic repositories.

Design alternatives:

In principle it would be possible to add a single large bloom filter
instead of embedding them in the index entry ids. However, this bloom
filter would necessarily incur additional random memory accesses and
thus slow things down overall.