Add functions `exp()`, `exp2()` & `round()`; fix minor readline issues

[Ma27]

• Added the functions exp(x)=e^x, exp2(x)=2^x, round(x) to the REPL
• readline-module now obtains defined functions from crate::lex rather than defining functions to highlight on its own.
• Updated rustyline to 0.9
• crate::lex now correctly detects functions with numbers in it (e.g. exp2).
• rustyline now always re-draws the REPL's current line. This isn't a big deal in our case IMHO and resolves a few highlighting bugs for me.
• rustyline now correctly distinguishes e from exp for highlighting.

[Ma27]

Applied both of your suggestions, thanks!