Formatter should not transform ASCII symbols to unicode by default

[paulcadman]

Formatting the following:

f : Nat -> Nat;
f _ := 1;

yields:

f : Nat → Nat;
f _ := 1;

Specifically the ASCII -> symbol has been transformed to the unicode →.

The user should be able to choose if they want these kind of non-whitespace transformations applied. By default they should not be applied.