Some kind of `FastString` instead of `string` for `ToString`

**The package I want to suggest the idea to**: AngouriMath

So there should be a class `FastString` which would be a bi-directional wrapper over `StringBuilder`, so that we could apply `+` operator to it. This class itself is mutable, so maybe it should be rather called `BiStringBuilder`.

The use of such a class would significantly (tens of times) boost `ToString()` of big expressions, and slighly speed up for small expressions.

Where to work:
`Sources/AngouriMath/Functions/Output/ToString`