Remove buggy fromString insertion with string interpolation

[andrevidela]

When using string interpolation the compiler inserts fromString calls at the wrong places. The details in #555 indicate that attempting to support fromString in interpolated string leads to inconsistent and unpredictable behaviour. A more moral argument is that interpolated strings are meant to concatenate strings specifically.

Steps to Reproduce

Use

"hello \{"world"} !"

Expected Behavior

The tree should return

"hello " ++ "world" ++ " !"

for the reason highlighted in #555

Observed Behavior

The tree is instead

fromString "hello" ++ fromString "world" ++ " !"

[andrevidela]

It turns out we cannot remove the fromString from the "world" call because we cannot guess what the type of its expression should be. In the example I assumed it was possible to assume each string literal in an interpolated slice evaluates to a string but that is not the case:

"Take this path: \{ show ("path" / "to" / "file") } for example"

Here the quotes within an interpolated slice must have the fromString call otherwise this expression wouldn't typecheck