Split core codes into detailed steps(parser, constructor, generator)

[romandev]

This change splits core codes into detailed steps in high-level.

• parser: The parser module is responsible for webidl parsing using
  the webidl2 parser internally. In this step, should create a unique
  type map(key: definition type name, value: AST) as an output.
• constructor: The constructor module is responsible for constructing
  definition object to generate binding code in generate module.
• generator: The generator module is responsible for generating actual
  native binding codes.

After this patch, we will be able to use it as follows:

const typeMap: types.IDLTypeMap = await parser.parse(idlFilePaths);
const definitions: types.IRDefinition[] =
    await constructor.construct(typeMap);
await generator.build(definitions);

ISSUE=#207