Improved Imenu and consistent behavior in multiline comments

• Improved Imenu and speedbar support
• More consistent electric indents and adaptive-fills inside multiline comments
• Chooses right executables for REPL, debugger, and other build tools
• The buffer name of REPL is now *Swift REPL [cmd-string]* where cmd-string is the command name such as swift
• Fixes beginning-of-defun hanging at the beginning of buffer
• Updates highlighting of class names for Swift 5.0