On nix-env -qa -f '<nixpkgs>', this reduces maximum RSS by 20970 KiB
and runtime by 0.8%. This is mostly because we're not parsing the hash
part as a hash anymore (just validating that it consists of base-32
characters).

Also, replace storePathToHash() by StorePath::hashPart().

Fixes #3618.

Fix master

E.g. instead of

  error: --- BuildError ----------------------------------------------- nix
  builder for '/nix/store/03nk0a3n8h2948k4lqfgnnmym7knkcma-foo.drv' failed with exit code 1
  error: --- Error ---------------------------------------------------- nix
  build of '/nix/store/03nk0a3n8h2948k4lqfgnnmym7knkcma-foo.drv' failed

we now get

  error: --- Error ---------------------------------------------------- nix
  builder for '/nix/store/03nk0a3n8h2948k4lqfgnnmym7knkcma-foo.drv' failed with exit code 1

These are now shown in the progress bar.

Closes #3577.

This means that 'throw Error({ ... ErrorInfo ... })' now works.

Use `std::string_view` in a few more places

Allow empty hash in derivations

This replaces the copy&paste with a helper function in hash.hh.

This function was used in only one place, where it could easily be
replaced by readDerivation() since it's not
performance-critical. (This function appears to have been modelled
after queryDerivationOutputs(), which exists only to make the garbage
collector faster.)

Add tests for pool.hh

Add an option to print the logs in a machine-readable format

libutils/hash: remove default encoding

Actually test nix-env with a remote store

Prelink static libraries into an object file

This combines the *.o into a big .o producing one translation unit.
This preserve our unused static initializers, as specified in the C++
standard:

  If no variable or function is odr-used from a given translation
  unit, the non-local variables defined in that translation unit may
  never be initialized (this models the behavior of an on-demand
  dynamic library).

Note that this is very similar to how the --whole-archive flag works.
One advantage of this is that users of the final .a library don’t have
to worry about specifying --whole-archive, or that we have unused
static initializers at all!

WSL1 doesn’t support i686-linux emulation, see https://github.com/microsoft/wsl/issues/2468

fetchTarball, fetchTree, and fetchGit all have *optional* hash attrs.
This means that we need to be careful with what we allow to avoid
accidentally making these defaults. When ‘hash = ""’ we assume the
empty hash is wanted.

follow up of https://github.com/NixOS/nix/pull/3544

This allows hash="" so that it can be used for debugging purposes. For
instance, this gives you an error message like:

  warning: found empty hash, assuming you wanted 'sha256:0000000000000000000000000000000000000000000000000000'
  hash mismatch in fixed-output derivation '/nix/store/asx6qw1r1xk6iak6y6jph4n58h4hdmbm-nix':
    wanted: sha256:0000000000000000000000000000000000000000000000000000
    got:    sha256:0fpfhipl9v1mfzw2ffmxiyyzqwlkvww22bh9wcy4qrfslb4jm429