runtime (gc_blocks.go): make sweep branchless

Instead of looping over each block, we can use bit hacks to operate on an entire state byte.
I deinterleaved the state bits in order to enable these tricks.

Sweep used to count free/freed allocations/blocks.
I managed to move/remove all of these counters:
- The free space is now calculated in buildFreeRanges by adding the range lengths.
- ReadMemStats counts freed objects by subtracting live objects from allocated objects.
- gcFreedBlocks was never necessary because MemStats.HeapAlloc is the same as MemStats.HeapInUse.

builder/sizes_test.go

@@ -42,9 +42,9 @@ func TestBinarySize(t *testing.T) {
 	// This is a small number of very diverse targets that we want to test.
 	tests := []sizeTest{
 		// microcontrollers
-		{"hifive1b", "examples/echo", 3808, 280, 0, 2268},
-		{"microbit", "examples/serial", 2790, 342, 8, 2272},
-		{"wioterminal", "examples/pininterrupt", 7327, 1493, 116, 6912},
+		{"hifive1b", "examples/echo", 3668, 280, 0, 2244},
+		{"microbit", "examples/serial", 2694, 342, 8, 2248},
+		{"wioterminal", "examples/pininterrupt", 7187, 1489, 116, 6888},
 
 		// TODO: also check wasm. Right now this is difficult, because
 		// wasm binaries are run through wasm-opt and therefore the