Merge key mapping with key bindings

[tompng]

Depends on #713
Fourth step of #708

Combines KeyActor::XXX_MAPPING(single byte to method symbol) with key_bindings(escape sequence to method symbol)

This will simplifies implementation of keys. Keyseq timeout waiting behavior becomes more similar to Readline.
We can remove key.instance_of?(String) check.
Reline::Key structure will be simple.

# before
EOF: Key{ char: nil, combined_char: nil, with_meta: false}
Normal key: Key{ char: number, combined_char: number, with_meta: false}
ESC-key: Key{ char: number, combined_char: number|0x80, with_meta: true}
Key-bindings: Key{ char: symbol, combined_char: symbol, with_meta: false}

# after
EOF: Key{ char: nil, combined_char: nil}
Other: Key{ char: string, method_symbol: string}

Future refactor

Key can contain string.
This means we can assign bracketed-pasted string to a key. bracketed_paste and quoted_insert will be simple.
Simplifies bracketed-paste and completion handling, undo-redo handling. Simplifies quoted_insert waiting_proc mechanism and fixes bug.

key.char = read_bracketed_paste if key.method_symbol == :bracketed_paste_start
key.char = read_next_valid_char if key.method_symbol == :quoted_insert
line_editor.update(key)

Implementing compress-meta will be significantly easy.

class Reline::KeyActor::Base
  def add_mappings(mappings)
    ...
    128.times do |key|
      ...
      add([27, key], meta_func)
      add([key | 0x80], meta_func) if compress_meta_enabled # Just add this line
    end
  end
end