Bug in string_of_type for generic placeholders

[gaurangsadekar]

Remembered this issue right now, I had found it yesterday.
This was working correctly earlier, before the pretty placeholders commit (8e5d6ea).

Expressions and incorrect outputs:

/\(x, y) => x;
/\ (x : A, y : A) : A = { { (x: A) } }

/\(x, y) => y;
/\ (x : A, y : A) : A = { { (y: A) } }

/\(x: B, y: A) => y;
/\ (x : A, y : A) : A = { { (y: A) } }

/\(x: T, y: U) => y;
/\ (x : A, y : A) : A = { { (y: A) } }

Seems like there is some problem with the naming of place holders, given that they always start with A.