Since there are some incompatible changes, it's better to bump the
major version number.

Hash always has a valid type

Only store hash in DerivationOutput for fixed output derivations

Allow PRECOMPILE_HEADERS in cross-compilation

In cross, CXX will look like aarch64-unknown-linux-gnu-g++. We could
run some command to check what kind of compiler it is, but for now we
can just check if g++ is anywhere in the string. I couldn’t find any
"ends with" for makefile, so it can be anywhere in CXX.

Reported by Kane York.

Get rid of `basicDerivation::findOutput`

It's a tiny function which is:

 - hardly worth abstrating over, and also only used once.

 - doesn't work once we get CA drvs

I rewrote the one callsite to be forwards compatable with CA
derivations, and also potentially more performant: instead of reading in
the derivation it can ust consult the SQLite DB in the common case.

Save changes made by "nix registry pin" to user registry

Separate concerns in `scanForReferences` with TeeSink

This also will make it easier to use a `HashModuloSink` instead for CA
derivations.

README: Fix link to hacking guide

The link was previously interpreted as if it were relative to the current file.

Allow relative paths in --store option

Add response body to network errors

The was Eelco's prefered logic, and it looks good to me!

Update chunkSize to the suggested value

This was a suggested course of action in a review in one of our earlier
commits, https://github.com/NixOS/nix/pull/3801#discussion_r457557079

Constant space `addToStoreFromDump` and deduplicate code

README: improve development docs

Correct bug, thoroughly document addToStoreSlow

Merge branch 'fix-and-document-addToStoreSlow' of github.com:obsidiansystems/nix into from-dump-stream

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

Due to https://github.com/NixOS/nix/issues/3841 we don't know how print
different messages for different verbosity levels.

Optimize `addToStoreSlow` and remove `TeeParseSink`

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>

Add newline to profile sourcing line #3393

fix make's impurity on /bin/sh

nix edit: call restoreSignals() before `execvp`-ing the $EDITOR

This is important when using tooling like BEAR to generate
compilation database since the used glibc version needs to match
for LD_PRELOAD to work. It might be also beneficial when building
on systems other than NixOS with nix develop since /bin/sh might
be not bash (which is what all nix devs use for testing).
This fix is not perfect because Makefile.config.in itself is
also build with make but strictly better than the status quo.