Sigil lexing
Make ElixirFlexLexer public for testing

JFlex defaults to the generated lexer being package-local, but that
complicates using nested packages as contexts for test, so use `%public`
to make the class and its constructors public.
Test all (non-bad) characters in YYINITIAL
Test all (non-bad) characters in BODY
Test all (non-bad) characters for INTERPOLATION
Order states in Elixir.flex

Make it easier to maintain code.
GROUP tests