Fixing highlighting of long character escape sequences.

Previously, '\U10ffff' was not highlighted correctly.
Removing julia-in-char.

It's an expensive function, and julia-in-string handles characters
anyway.
Fontify strings and chars as strings, not keywords.

This is faster and less code.
Silencing byte-compiler warning.

This is warning is harmless (I'm not aware of any plans to remove
`font-lock-syntactic-keywords`) but it has caused concern amongst users.

See https://github.com/JuliaLang/julia/pull/9042
Highlight triple quoted strings.
Merge pull request #9152 from Wilfred/wh/triple_quoted_highlighting

julia-mode.el: Highlight triple-quoted strings and characters.