hreflect: Cache reflect method lookups used in collections.Where and others

hreflect: Cache reflect method lookups used in collections.Where and others

We already cached the method index on the struct, but caching the resolved `reflect.Method` itself saves us from having to do another lookup on each call, which is escpeciall import in the hot path used by collections.Where and otherrs:

```bash
                                        │ master.bench │    fix-reflectmethodcache.bench     │
                                        │    sec/op    │    sec/op     vs base               │
WhereSliceOfStructPointersWithMethod-10   592.2µ ± ∞ ¹   390.1µ ± ∞ ¹  -34.14% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95

                                        │  master.bench  │     fix-reflectmethodcache.bench     │
                                        │      B/op      │     B/op       vs base               │
WhereSliceOfStructPointersWithMethod-10   205.14Ki ± ∞ ¹   64.52Ki ± ∞ ¹  -68.55% (p=0.029 n=4)
¹ need >= 6 samples for confidence interval at level 0.95

                                        │ master.bench │    fix-reflectmethodcache.bench     │
                                        │  allocs/op   │  allocs/op    vs base               │
WhereSliceOfStructPointersWithMethod-10   9.003k ± ∞ ¹   4.503k ± ∞ ¹  -49.98% (p=0.029 n=4)
````