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).
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 didn't work anymore since decompression was only done in the
non-coroutine case.
Decompressors are now sinks, just like compressors.
Also fixed a bug in bzip2 API handling (we have to handle BZ_RUN_OK
rather than BZ_OK), which we didn't notice because there was a missing
'throw':
if (ret != BZ_OK)
CompressionError("error while compressing bzip2 file");
Fixes
error: getting status of '/nix/store/j8p0vv89k1pf0cn7kmfsdcs7bshwga1i-firefox-52.7.2esr/share/icons/hicolor/48x48/apps/firefox.png': No such file or directory
https://github.com/NixOS/nix/issues/1934
Also improve error message on directory/non-directory collisions.
