runtime: sizespecializedmalloc experiment

[matloob]

This issue is for tracking work on the sizespecializedmalloc GOEXPERIMENT. The goal is to specialize variants of the mallocgc for specific small sizeclasses to see if they could bring a performance improvement.

[gopherbot]

Change https://go.dev/cl/773260 mentions this issue: runtime: zero invalid bits of allocCache in sizespecializedmalloc

[gopherbot]

Change https://go.dev/cl/772540 mentions this issue: runtime: consolidate tiny sizespecializedmalloc functions

[gopherbot]

Change https://go.dev/cl/775980 mentions this issue: runtime: add microbenchmarks for sizespecializedmalloc

[gopherbot]

Change https://go.dev/cl/776020 mentions this issue: runtime: make nelems a constant in sizespecializedmalloc

[gopherbot]

Change https://go.dev/cl/776021 mentions this issue: runtime: in nextFreeFastStub refillAllocCache when needed

[gopherbot]

Change https://go.dev/cl/776740 mentions this issue: runtime: remove specialized classes larger than 128 bytes