all: rewrite goroutine lowering

Before this commit, goroutine support was spread through the compiler.
This commit changes this support, so that the compiler itself only
generates simple intrinsics and leaves the real support to a compiler
pass that runs as one of the TinyGo-specific optimization passes.

The biggest change, that was done together with the rewrite, was support
for goroutines in WebAssembly for JavaScript. The challenge in
JavaScript is that in general no blocking operations are allowed, which
means that programs that call time.Sleep() but do not start goroutines
also have to be scheduled by the scheduler.

src/runtime/runtime_wasm.go

@@ -6,11 +6,9 @@ import (
 	"unsafe"
 )
 
-type timeUnit int64
+type timeUnit float64 // time in milliseconds, just like Date.now() in JavaScript
 
-const tickMicros = 1
-
-var timestamp timeUnit
+const tickMicros = 1000000
 
 //go:export io_get_stdout
 func io_get_stdout() int32
@@ -32,21 +30,27 @@ func _start() {
 //go:export cwa_main
 func cwa_main() {
 	initAll() // _start is not called by olin/cwa so has to be called here
-	mainWrapper()
+	callMain()
 }
 
 func putchar(c byte) {
 	resource_write(stdout, &c, 1)
 }
 
-func sleepTicks(d timeUnit) {
-	// TODO: actually sleep here for the given time.
-	timestamp += d
+//go:export go_scheduler
+func go_scheduler() {
+	scheduler()
 }
 
-func ticks() timeUnit {
-	return timestamp
-}
+const asyncScheduler = true
+
+// This function is called by the scheduler.
+// Schedule a call to runtime.scheduler, do not actually sleep.
+//go:export runtime.sleepTicks
+func sleepTicks(d timeUnit)
+
+//go:export runtime.ticks
+func ticks() timeUnit
 
 // Abort executes the wasm 'unreachable' instruction.
 func abort() {