Optimize memory allocations for sites matrix vector stores

Optimize memory allocations for sites matrix vector stores

By

* Caching common sites matrix setups (e.g. the single site in single site setups).
* Adding a fast path to IntSets.HasAnyVector for the common case of single vector input.

```
AssembleDeepSiteWithManySections/depth=3/sectionsPerLevel=2/pagesPerSection=100-10   31.62m ± 46%   30.68m ± 42%  ~ (p=0.310 n=6)

                                                                                   │ master.bench │          perfcommon.bench          │
                                                                                   │     B/op     │     B/op      vs base              │
AssembleDeepSiteWithManySections/depth=3/sectionsPerLevel=2/pagesPerSection=100-10   31.98Mi ± 0%   31.24Mi ± 0%  -2.30% (p=0.002 n=6)

                                                                                   │ master.bench │         perfcommon.bench          │
                                                                                   │  allocs/op   │  allocs/op   vs base              │
AssembleDeepSiteWithManySections/depth=3/sectionsPerLevel=2/pagesPerSection=100-10    460.9k ± 0%   419.9k ± 0%  -8.90% (p=0.002 n=6)
````