Format the AST output

[Keluaa]

We should run a formatter on the String result of sprint(io, expr).
JuliaFormatting.jl and Runic.jl may be useless for this task, as there they both assume user-written code, but printing an Expr to String will return almost clean Julia code.

For this to work nicerly, get_code should return a String instead of Expr, then the formatting would be applied as a cleanup stage.

Writing a custom formatter/prettifier would be nice:

• re-indent some lines, especially those with macros placed in front of blocks or loops
• put empty lines between block, for, if and elseif if they are large
• re-indent, or convert to short ternary form a ? b : c if the if is small
• remove useless parenthesis around functions placed within Expr (e.g. :($f(a, b)) -> "(Mod.f)(a, b)" -> "Mod.f(a, b)")