runtime: simpler and faster GC

Implement the design described in:
https://docs.google.com/document/d/1v4Oqa0WwHunqlb8C3ObL_uNQw3DfSY-ztoA-4wWbKcg/pub

Summary of the changes:
GC uses "2-bits per word" pointer type info embed directly into bitmap.
Scanning of stacks/data/heap is unified.
The old spans types go away.
Compiler generates "sparse" 4-bits type info for GC (directly for GC bitmap).
Linker generates "dense" 2-bits type info for data/bss (the same as stacks use).

Summary of results:
-1680 lines of code total (-1000+ in mgc0.c only)
-25% memory consumption
-3-7% binary size
-15% GC pause reduction
-7% run time reduction

LGTM=khr
R=golang-codereviews, rsc, christoph, khr
CC=golang-codereviews, rlh
https://golang.org/cl/106260045

src/pkg/runtime/export_test.go

@@ -62,6 +62,9 @@ func ParForIters(desc *ParFor, tid uint32) (uint32, uint32) {
 	return uint32(begin), uint32(end)
 }
 
+//go:noescape
+func GCMask(x interface{}) []byte
+
 func testSchedLocalQueue()
 func testSchedLocalQueueSteal()