Fixed VimL literal string lexer.

The only escape string in a vim literal string (single quotes) is `''`
which is expanded to a single, single-quote.

This was not the behaviour of the previous implementation and this
caused incorrect syntax highlighting as a result.
Detect Objective-C snippets if there is a NSNumber object
get_lexer_by_name: handle edge case when _alias is None
Nix: add null constant and improve text highlighting
Haskell: restrict char literals to 1 character if no escapes
Merged in iElectric/pygments-main (pull request #301)

Nix: add null constant and improve text highlighting
merge with iElectric/pygments-main
Merged in jaingaurav2/pygments-main/objc (pull request #299)

Detect Objective-C snippets if there is a NSNumber object
Merged in andyops/pygments-main/fixed-vim-literal-strings (pull request #298)

Fixed VimL literal string lexer.
merge