If the `throw` is reached, this means that execvp into `ssh` wasn’t
successful. We can hint at a usual problem, which is a missing `ssh`
executable.

Test with:

```
env PATH= ./result/bin/nix-copy-closure --builders '' unusedhost
```

and the bash version with

```
env PATH= ./result/bin/nix-copy-closure --builders '' localhost
```

1.81% -> 0.56%

From 1.03% to 0.19% of the runtime of 'nix-instantiate "<nixpkgs>" -A
texlive.combined.scheme-full --dry-run'.

Most functions now take a StorePath argument rather than a Path (which
is just an alias for std::string). The StorePath constructor ensures
that the path is syntactically correct (i.e. it looks like
<store-dir>/<base32-hash>-<name>). Similarly, functions like
buildPaths() now take a StorePathWithOutputs, rather than abusing Path
by adding a '!<outputs>' suffix.

Note that the StorePath type is implemented in Rust. This involves
some hackery to allow Rust values to be used directly in C++, via a
helper type whose destructor calls the Rust type's drop()
function. The main issue is the dynamic nature of C++ move semantics:
after we have moved a Rust value, we should not call the drop function
on the original value. So when we move a value, we set the original
value to bitwise zero, and the destructor only calls drop() if the
value is not bitwise zero. This should be sufficient for most types.

Also lots of minor cleanups to the C++ API to make it more modern
(e.g. using std::optional and std::string_view in some places).

In particular, this enables HTTP/2 support in reqwest, which is a lot
more efficient.

The FunctionCallTrace object consumes a few hundred bytes of stack
space, even when tracing is disabled. This was causing stack overflows:

  $ nix-instantiate '<nixpkgs> -A texlive.combined.scheme-full --dry-run
  error: stack overflow (possible infinite recursion)

This is with the default stack size of 8 MiB.

Putting the object on the heap reduces stack usage to < 5 MiB.

This is no longer needed since we're not using POSIX locks anymore.

(cherry picked from commit 3392f1b7)

(cherry picked from commit 96c6b08e)

(cherry picked from commit 63c5c91c)

(cherry picked from commit 8beedd44)

(cherry picked from commit fb692e5f)

This allows writing attribute lookups as

    if (auto name = value.attrs->get(state.sName))
      ...

(cherry picked from commit f216c76c)

It's now regenerated when util.hh changes, and is ordered after
config.h to fix a race.

(cherry picked from commit d0a769cb)

(cherry picked from commit a0de58f4)

(cherry picked from commit f70434b1)

(cherry picked from commit 15a16e5c)

This reverts commit 2b761d5f.

Also *really* make fmt() take arguments by reference.

doc/manual: add note to `allowSubstitutes` advanced attribute

https://hydra.nixos.org/build/107724274

This makes it possible to reference single attribute definitions,
for pointing people to their exact definition.

Fix typos