Fix quasiquote free variable interpolation

[ners]

Before this PR, HNix provided an implementation for interpolating free variables as Nix expressions. This was not called correctly, because the types did not match: it looked for NExprLoc instead of NExpr.

This PR fixes interpolation such that the following code works as expected:

let foo = "foo" :: String
[nix|
{
  foo = foo;
}
|]

produces:

{
  foo = "foo";
}

To do this, we fix the ToExpr typeclass to generate NExpr. We also provide some additional instances.

The diff also includes some lines that have been reformatted when running Brittany on the source file.

[Anton-Latukha]

Thank you for fixing it.