Adds experimental write support and implements on gojs (#970)

This adds writefs.FS, allowing functions to create and delete files.
This begins by implementing them on `GOARCH=js GOOS=wasm`. The current
status is a lot farther than before, even if completing write on WASI is
left for a later PR (possibly by another volunteer).

Signed-off-by: Adrian Cole <adrian@tetrate.io>

internal/gojs/state.go

@@ -144,8 +144,12 @@ func storeRef(ctx context.Context, v interface{}) goos.Ref { //nolint
 	} else if _, ok := v.(string); ok {
 		id := getState(ctx).values.increment(v)
 		return goos.ValueRef(id, goos.TypeFlagString)
+	} else if i32, ok := v.(int32); ok {
+		return toFloatRef(float64(i32))
 	} else if u32, ok := v.(uint32); ok {
 		return toFloatRef(float64(u32))
+	} else if i64, ok := v.(int64); ok {
+		return toFloatRef(float64(i64))
 	} else if u64, ok := v.(uint64); ok {
 		return toFloatRef(float64(u64))
 	} else if f64, ok := v.(float64); ok {
@@ -261,6 +265,15 @@ func toInt64(arg interface{}) int64 {
 	return int64(arg.(float64))
 }
 
+func toUint64(arg interface{}) uint64 {
+	if arg == goos.RefValueZero || arg == undefined {
+		return 0
+	} else if u, ok := arg.(uint64); ok {
+		return u
+	}
+	return uint64(arg.(float64))
+}
+
 func toUint32(arg interface{}) uint32 {
 	if arg == goos.RefValueZero || arg == undefined {
 		return 0