Advice: I think there are many ways to make go-swiss better

[ahfuzhang]

Give some advice about this lib:

1. ctrl byte is everything, so no need to set kv when delete.
   those lines are not need:
   

   swiss/map.go

   Line 141 in c0064a0

   m.groups[g].keys[s] = key

swiss/map.go

Line 190 in c0064a0

m.groups[g].keys[s] = k

swiss/map.go

Line 246 in c0064a0

for i := range m.groups {

2.when init ctrl bytes, we can use simd or unsafe code, not one by one.
see:

swiss/map.go

Line 69 in c0064a0

for i := range m.ctrl {

3.If the group's ctrl bytes are all empty, Iter() can be faster.
see:

swiss/map.go

Line 220 in c0064a0

for n := 0; n < len(groups); n++ {

we can add: if metaMatchEmpty(&m.ctrl[g])==0xffff{continue;}

4.Let ctrl bytes slice address is align to 32, so we can use _mm_load_128() to replace __mm_loadu_128():
see:

swiss/map.go

Line 64 in c0064a0

ctrl: make([]metadata, groups),

alloc more 32 bytes, and use unsafe code to choose a start address of 128bit alignment.

5. If group count is power of 2, not need to use if to check range.
   see:

   swiss/map.go

   Line 96 in c0064a0

   if g >= uint32(len(m.groups)) {

   
   g = g & 0xffff // if group count is 65536

6.try groupSize=32, and use avx2 to match 32 bytes each time.

7. XXHash might be better than golang map hasher.

[ahfuzhang]

I think I understand the purpose of those lines on advice 1:

when the content of the object is the same but the pointer is different, replace it with the current object. And when delete, we should set to null for gc.

[ahfuzhang]

However, for ultimate performance optimization, it is recommended to provide a value type version. Then remove the operations that are valid for reference types but not valid for value types.