napi: String API improvements (nodejs#83)

 - Make string API names consistent
 - Make string API coercion behavior consistent
 - Switch from number-remaining to number-copied results
 - Add UTF-16 string APIs
 - Add coercion APIs for number and bool
 - Update tests for API changes

src/node_api_helpers.h

@@ -87,28 +87,24 @@ namespace Napi {
    public:
     inline explicit Utf8String(napi_value from) :
         length_(0), str_(str_st_) {
-      if (from != NULL) {
+      if (from != nullptr) {
         napi_env env;
         napi_get_current_env(&env);
-        napi_value string;
+        napi_value string = nullptr;
         napi_coerce_to_string(env, from, &string);
-        if (string != NULL) {
-          int len;
-          napi_get_string_length(env, string, &len);
-          size_t utf8len = 3 * len + 1;
-          assert(utf8len <= INT_MAX);
-          if (utf8len > sizeof(str_st_)) {
-            str_ = new char[utf8len];
+        if (string != nullptr) {
+          napi_get_value_string_utf8_length(env, string, &length_);
+          int bufsize = length_ + 1;
+          if (static_cast<size_t>(bufsize) > sizeof(str_st_)) {
+            str_ = new char[bufsize];
             assert(str_ != 0);
           }
-          napi_get_string_utf8(env, string, str_,
-            static_cast<int>(utf8len), &length_);
-          str_[length_] = '\0';
+          napi_get_value_string_utf8(env, string, str_, bufsize, nullptr);
         }
       }
     }
 
-    inline int length() const {
+    inline size_t length() const {
       return length_;
     }
 
@@ -431,7 +427,7 @@ namespace Napi {
       napi_get_current_env(&env);
 
       napi_value s;
-      napi_create_string(env, ErrorMessage(), &s);
+      napi_create_string_utf8(env, ErrorMessage(), -1, &s);
 
       napi_value argv[1];
       napi_create_error(env, s, argv);