wasm: use wasi ABI for basic startup/stdout

This allows TinyGo-built binaries to run under wasmtime, for example:

    tinygo build -o test.wasm -no-debug -target=wasm examples/test
    wasmtime run test.wasm 0

src/runtime/runtime_wasm.go

@@ -2,35 +2,43 @@
 
 package runtime
 
+import "unsafe"
+
 type timeUnit float64 // time in milliseconds, just like Date.now() in JavaScript
 
 const tickMicros = 1000000
 
-//go:export io_get_stdout
-func io_get_stdout() int32
-
-//go:export resource_write
-func resource_write(id int32, ptr *uint8, len int32) int32
-
-var stdout int32
-
-func init() {
-	stdout = io_get_stdout()
+// Implements __wasi_ciovec_t and __wasi_iovec_t.
+type wasiIOVec struct {
+	buf    unsafe.Pointer
+	bufLen uint
 }
 
-//go:export _start
+//go:wasm-module wasi_unstable
+//export fd_write
+func fd_write(id uint32, iovs *wasiIOVec, iovs_len uint, nwritten *uint) (errno uint)
+
+//export _start
 func _start() {
 	initAll()
-}
-
-//go:export cwa_main
-func cwa_main() {
-	initAll() // _start is not called by olin/cwa so has to be called here
 	callMain()
 }
 
+// Using global variables to avoid heap allocation.
+var (
+	putcharBuf   = byte(0)
+	putcharIOVec = wasiIOVec{
+		buf:    unsafe.Pointer(&putcharBuf),
+		bufLen: 1,
+	}
+)
+
 func putchar(c byte) {
-	resource_write(stdout, &c, 1)
+	// write to stdout
+	const stdout = 1
+	var nwritten uint
+	putcharBuf = c
+	fd_write(stdout, &putcharIOVec, 1, &nwritten)
 }
 
 var handleEvent func()