Change "while evaluating <fun>" to "while *calling*" in trace

The old way was not correct.

Here is an example:
```
 $ nix-instantiate --eval --expr 'let x = a: throw "asdf"; in x 1' --show-trace
error: asdf

       … while evaluating 'x'

       at «string»:1:9:

            1| let x = a: throw "asdf"; in x 1
             |         ^

       … from call site

       at «string»:1:29:

            1| let x = a: throw "asdf"; in x 1
             |                             ^
```
and yet also:
```
 $ nix-instantiate --eval --expr 'let x = a: throw "asdf"; in x' --show-trace
<LAMBDA>
```

Here is the thing: in both cases we are evaluating `x`!

Nix is a higher-order languages, and functions are a sort of value. When
we write `x = a: ...`, `a: ...` is the expression that `x` is being
defined to be, and that is already a value. Therefore, we should *never*
get an trace that says "while evaluating `x`", because evaluating `a:
...` is *trival* and nothing happens during it!

What is actually happening here is we are applying `x` and evaluating
its *body* with arguments substituted for parameters. I think the
simplest way to say is just "while *calling* `x`", and so that is what I
changed it to.

0 files changed, 0 insertions, 0 deletions