Better String Representation of Composite Functions

Similar to `setSource` but for 2 functions, `setCompositeRepr` creates a
string representation for the composed function in this style
```
function f (...)  {
//stuff
}

OF

function g (...)  {
//stuff
}
```
More specifically, its the string represetnation of `f` + new
lines + `"OF"` + new lines + the string representation of `g`, for the
function `compose(f, g)`.