https://hydra.nixos.org/build/107467517

Seems that on i686-linux, gcc and rustc disagree on how to return
1-word structs: gcc has the caller pass a pointer to the result, while
rustc has the callee return the result in a register. Work around this
by using a bare pointer.

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

Also, fetchGit now runs in O(1) memory since we pipe the output of
'git archive' directly into unpackTarball() (rather than first reading
it all into memory).

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.

This is no longer needed.

This was the last function using a shell script, so this allows us to
get rid of tar, coreutils, bash etc.

https://hydra.nixos.org/build/107457009

Downloader: Log configured CA file

Fix sandbox fallback settings

Ensure enough space in attrset bindings

Fixes #3239.

E.g.

  $ nix-build '<nixpkgs>' -A hello --experimental-features no-url-literals
  error: URL literals are disabled, at /nix/store/vsjamkzh15r3c779q2711az826hqgvzr-nixpkgs-20.03pre194957.bef773ed53f/nixpkgs/pkgs/top-level/all-packages.nix:1236:11

Helps with implementing https://github.com/NixOS/rfcs/pull/45.

Document builtins.placeholder

This allows it to be set in nix.conf.