The term "native module" dates back to some of the oldest code
in the code base. Within the context of Node.js core it usually
refers to modules that are native to Node.js (e.g. fs, http),
but it can cause confusion for people who don't work on this
part of the code base, as "native module" can also refer to
native addons - which is even the case in some of the API
docs and error messages.

This patch tries to make the usage of these terms more consistent.
Now within the context of Node.js core:

- JavaScript scripts that are built-in to Node.js are now referred
  to as "built-in(s)". If they are available as modules,
  they can also be referred to as "built-in module(s)".
- Dynamically-linked shared objects that are loaded into
  the Node.js processes are referred to as "addons".

We will try to avoid using the term "native modules" because it could
be ambiguous.

Changes in this patch:

File names:
- node_native_module.h -> node_builtins.h,
- node_native_module.cc -> node_builtins.cc

C++ binding names:
- `native_module` -> `builtins`

`node::Environment`:
- `native_modules_without_cache` -> `builtins_without_cache`
- `native_modules_with_cache` -> `builtins_with_cache`
- `native_modules_in_snapshot` -> `builtins_in_cache`
- `native_module_require` -> `builtin_module_require`

`node::EnvSerializeInfo`:
- `native_modules` -> `builtins

`node::native_module::NativeModuleLoader`:
- `native_module` namespace -> `builtins` namespace
- `NativeModuleLoader` -> `BuiltinLoader`
- `NativeModuleRecordMap` -> `BuiltinSourceMap`
- `NativeModuleCacheMap` -> `BuiltinCodeCacheMap`
- `ModuleIds` -> `BuiltinIds`
- `ModuleCategories` -> `BuiltinCategories`
- `LoadBuiltinModuleSource` -> `LoadBuiltinSource`

`loader.js`:
- `NativeModule` -> `BuiltinModule` (the `NativeModule` name used in
  `process.moduleLoadList` is kept for compatibility)

And other clarifications in the documentation and comments.