Previously, the build system used uname(1) output when it wanted to
check the operating system it was being built for, which meant that it
didn't take into-account cross-compilation when the build and host
operating systems were different.

To fix this, instead of consulting uname output, we consult the host
triple, specifically the third "kernel" part.

For "kernel"s with stable ABIs, like Linux or Cygwin, we can use a
simple ifeq to test whether we're compiling for that system, but for
other platforms, like Darwin, FreeBSD, or Solaris, we have to use a
more complicated check to take into account the version numbers at the
end of the "kernel"s.  I couldn't find a way to just strip these
version numbers in GNU Make without shelling out, which would be even
more ugly IMO.  Because these checks differ between kernels, and the
patsubst ones are quite fiddly, I've added variables for each host OS
we might want to check to make them easier to reuse.

Linux is (as far as I know) the only mainstream operating system that
requires linking with libdl for dlopen.  On BSD, libdl doesn't exist,
so on non-FreeBSD BSDs linking will currently fail.  On macOS, it's
apparently just a symlink to libSystem (macOS libc), presumably
present for compatibility with things that assume Linux.

So the right thing to do here is to only add -ldl on Linux, not to add
it for everything that isn't FreeBSD.

We're not producing source tarballs anymore so this has been
bitrotting.

Do idiomatic C++ copy and move constructors for a few things, so
wrapping structs' defaults can work.

As of Rust 2018, macro_use is no longer required in most circumstances.
I think it is generally a good idea to remove these when not needed, to
stop them from polluting the crate's global namespace.
https://doc.rust-lang.org/edition-guide/rust-2018/macros/macro-changes.html#macro_rules-style-macros

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.

This is a hack to fix the build on macOS, which was failing because
libnixrust.a contains compiler builtins that clash with
libclang_rt.osx.a. There's probably a better solution...

https://hydra.nixos.org/build/107473280

https://hydra.nixos.org/build/107466992

In particular, these are emitted by 'git archive' (in fetchGit).

We can now convert Rust Errors to C++ exceptions. At the Rust->C++ FFI
boundary, Result<T, Error> will cause Error to be converted to and
thrown as a C++ exception.