This isn't used anywhere except in the configure script of the Perl
bindings. I've changed the latter to use the C++ API's Settings object
at runtime.

Path is null when not known statically.

This is only used by hydra-queue-runner and it's better to implement
it there.

we don’t need a full storepath for a fixedoutput derivation. So just
putting the ingestion method + the hash is sufficient.

Not a regular git revert as there have been many merges and things.

This will make it easier to reason about the hash encoding and switch to
SRI everywhere where possible.

This does a few enums; the rest will be gotten in subsequent commits.

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).

This reverts commit 717e821b. It's
much more convenient to do 'make OPTIMIZE=0'.

As is normal for autoconf-based projects.

For example, it is a common use case to do

    ./configure CXXFLAGS=-O0

This did not work for nix until now, because the `CXXFLAGS=` declaration
would unconditionally erase what the user had specified.

The custom `OPTIMIZE` flag is removed, but the default `-O3` is retained;
autoconf would default to `-g -O2` by default otherwise as documented on:

https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C-Compiler.html
https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C_002b_002b-Compiler.html

Allow global config settings to be defined in multiple Config
classes. For example, this means that libutil can have settings and
evaluator settings can be moved out of libstore. The Config classes
are registered in a new GlobalConfig class to which config files
etc. are applied.

Relevant to https://github.com/NixOS/nix/issues/2009 in that it
removes the need for ad hoc handling of useCaseHack, which was the
underlying cause of that issue.

Relevant RFC: NixOS/rfcs#4

$ ag -l | xargs sed -i -e "/\"/s/’/'/g;/\"/s/‘/'/g"

Also simplify the Hash API.

Fixes #1437.

This makes all config options self-documenting.

Unknown or unparseable config settings and --option flags now cause a
warning.

Somehow this came back after d1da6967.

Fixes #1283.

Also get rid of Settings::processEnvironment(), it appears to be
useless.

This reverts commit f78126bf. There
really is no need for such a massive change...