Unite key bindings, key mapping and multibyte buffer

[tompng]

What is this?

KeyActor, key_bindings(key_stroke.rb) and multibyte_buffer(LineEditor) can be united. This will reduce complexity.
This pull request is a goal of this refactor. I'm planning to open several separate pull requests.

What can be solved?

• LineEditor does not need to handle 3 types of input: part of multibyte character, symbol key, ascii byte assigned to a method.
• Complex read_2nd_character_of_key_sequence can be removed.
• Reline can't correctly handle conflicting key binding specified in inputrc now. It can be fixed.

What to do

• Improve KeyStroke
  • Matching algorithm needs to be faster because the total entry will increase (default_key_bindings(size=25) → key_bindings + key_mappings(size=256))
  • Should have three state(:matching | :matched | :matching_matched) but the last one is missing now
  • Delete ESC-specific read_2nd_character_of_key_sequence and implement generic one
• Move multibyte_buffer from LineEditor to key recognizing layer
• Simplify Reline::Key structure, no mixing symbol and integer