Improved comments and defuns

Added comment style option.

Now we can choose multiline comment styles.

Java style:

/**
 * Foo
 * Bar
 */

Swift Quick Help style:

/**
 Foo
 Bar
 */

https://developer.apple.com/library/content/documentation/Xcode/Reference/xcode_markup_formatting_ref/index.html

Quick Help style is the default.

Single line comment now inherits spaces from the previous line.

/// - aaa
///   - bbb // When M-j (indent-new-comment-line) is pressed here,
///   // triple spaces will be inserted after slashes.

Improved functions related to defuns. More robust and sensible behaviors.