wasi: optimizes args/environ parsing (#885)

While most compilers will only read args/environ once, tools like WAGI
make heavy use of environment, possibly dozens of long variables. This
optimizes both args and environ for this reason and also to setup for
optimizing other functions.

Here are the notable changes:
* eagerly coerce to byte slices instead of strings
* re-use null terminated length for writing values
* avoid loops that call mem.WriteXXX internally

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

config.go

@@ -462,9 +462,9 @@ type moduleConfig struct {
 	nanotime           *sys.Nanotime
 	nanotimeResolution sys.ClockResolution
 	nanosleep          *sys.Nanosleep
-	args               []string
+	args               [][]byte
 	// environ is pair-indexed to retain order similar to os.Environ.
-	environ []string
+	environ [][]byte
 	// environKeys allow overwriting of existing values.
 	environKeys map[string]int
 	// fs is the file system to open files with
@@ -492,19 +492,30 @@ func (c *moduleConfig) clone() *moduleConfig {
 // WithArgs implements ModuleConfig.WithArgs
 func (c *moduleConfig) WithArgs(args ...string) ModuleConfig {
 	ret := c.clone()
-	ret.args = args
+	ret.args = toByteSlices(args)
 	return ret
 }
 
+func toByteSlices(strings []string) (result [][]byte) {
+	if len(strings) == 0 {
+		return
+	}
+	result = make([][]byte, len(strings))
+	for i, a := range strings {
+		result[i] = []byte(a)
+	}
+	return
+}
+
 // WithEnv implements ModuleConfig.WithEnv
 func (c *moduleConfig) WithEnv(key, value string) ModuleConfig {
 	ret := c.clone()
 	// Check to see if this key already exists and update it.
 	if i, ok := ret.environKeys[key]; ok {
-		ret.environ[i+1] = value // environ is pair-indexed, so the value is 1 after the key.
+		ret.environ[i+1] = []byte(value) // environ is pair-indexed, so the value is 1 after the key.
 	} else {
 		ret.environKeys[key] = len(ret.environ)
-		ret.environ = append(ret.environ, key, value)
+		ret.environ = append(ret.environ, []byte(key), []byte(value))
 	}
 	return ret
 }
@@ -602,21 +613,29 @@ func (c *moduleConfig) WithRandSource(source io.Reader) ModuleConfig {
 
 // toSysContext creates a baseline wasm.Context configured by ModuleConfig.
 func (c *moduleConfig) toSysContext() (sysCtx *internalsys.Context, err error) {
-	var environ []string // Intentionally doesn't pre-allocate to reduce logic to default to nil.
+	var environ [][]byte // Intentionally doesn't pre-allocate to reduce logic to default to nil.
 	// Same validation as syscall.Setenv for Linux
 	for i := 0; i < len(c.environ); i += 2 {
 		key, value := c.environ[i], c.environ[i+1]
-		if len(key) == 0 {
+		keyLen := len(key)
+		if keyLen == 0 {
 			err = errors.New("environ invalid: empty key")
 			return
 		}
-		for j := 0; j < len(key); j++ {
-			if key[j] == '=' { // NUL enforced in NewContext
+		valueLen := len(value)
+		result := make([]byte, keyLen+valueLen+1)
+		j := 0
+		for ; j < keyLen; j++ {
+			if k := key[j]; k == '=' { // NUL enforced in NewContext
 				err = errors.New("environ invalid: key contains '=' character")
 				return
+			} else {
+				result[j] = k
 			}
 		}
-		environ = append(environ, key+"="+value)
+		result[j] = '='
+		copy(result[j+1:], value)
+		environ = append(environ, result)
 	}
 
 	return internalsys.NewContext(