[WIP] variable: return native Go pointers to underlying bpf map values

[ti-mo]

Pending the proposal in golang/go#70224, here's the current API proposal for returning Go pointers to 'off-heap' bpf map memory:

func MemoryPointer[T any](mm *Memory, off uint64) (*T, error)
func VariablePointer[T any](v *Variable) (*T, error)

We need a bit of help from the Go runtime to manage the lifecycle of the underlying memory mapping. Currently, this mmaps over a piece of Go heap to allow the GC to track pointers into the bpf map memory, obviating the need for tying the mmap's lifetime to an ebpf.Memory. Instead, any Go pointers into the mmap will keep it alive, removing the risk of use-after-free.

Clearly, this involves some dark arts, so we're not comfortable pushing this on users since Collection.Variables is populated by default. Any change in the runtime or allocator may break this, and the fallout means Go programs segfaulting without any hint as to why, which is not acceptable.