julia-mode.el: Use the tex input mode

[Wilfred]

A couple of kind elisp hackers were reviewing julia-mode.el, and pointed out that we could simply reuse TeX's input mode in elisp. This would save us needing to copy everything from the REPL's definitions.

(Sorry for opening all these tickets, I have fixes in the works for them!)

[JeffBezanson]

That would be great; don't worry, we appreciate it!

[stevengj]

We already discussed this on the mailing list at some point.

Drawbacks of tex input mode:

• The tex input mode does not use the same Unicode codepoints as the REPL (e.g. for \hbar), which could lead to inscrutable errors when going between the REPL and emacs.
• Emacs tex input does not support exactly the same set of LaTeX macros as the REPL—it looks like emacs supports many fewer macros (e.g. \mscrS for 𝒮 from the unicode-math package is not supported by tex input).
• It is nicer to have identical tab-completion TeX-input behavior for different Julia environments (REPL, IJulia, Emacs, vim, ...).
• Switching to/from tex input mode is a lot more keystrokes than just hitting tab. (And having the TeX input method turned on all the time is intrusive and annoying whenever you type a backslash...bordering on unusable since \ is a common operator in Julia code.)

Copying the definitions from the REPL to julia-mode.el is pretty painless because it is automated by a script.

[Wilfred]

OK, I'm pretty persuaded by the reasons there. I'll add a comment to julia-mode.el to explain this reasoning.