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.

This is not used anywhere.

This allows distinguishing between an empty value and no value.

A corrupt entry in .links prevents adding a fixed version of that file
to the store in any path.  The user experience is that corruption
present in the store 'spreads' to new paths added to the store:

(With store optimisation enabled)

1. A file in the store gets corrupted somehow (eg: filesystem bug).
2. The user tries to add a thing to the store which contains a good copy
   of the corrupted file.
3. The file being added to the store is hashed, found to match the bad
   .links entry, and is replaced by a link to the bad .links entry.
   (The .links entry's hash is not verified during add -- this would
   impose a substantial performance burden.)
4. The user observes that the thing in the store that is supposed to be
   a copy of what they were trying to add is not a correct copy -- some
   files have different contents!  Running "nix-store --verify
   --check-contents --repair" does not fix the problem.

This change makes "nix-store --verify --check-contents --repair" fix
this problem.  Bad .links entries are simply removed, allowing future
attempts to insert a good copy of the file to succeed.

manual: nix-shell: Elaborate on using `-p` with expressions

Replace $TMPDIR with $TEST_ROOT in tests/fetchurl.sh

Fix extra semicolons warnings

Thanks @bhipple for catching!

This documents the outcome of the change in
https://github.com/NixOS/nix/issues/454:

> We can also automatically add parentheses in the generated
> `buildInputs`, so you can type `nix-shell -p "expr"`
> instead of `"(expr").

De-duplicate struct PrimOp forward declaration

$TMPDIR isn't necessarily set and would cause this test to fail.

gitignore /precompiled-headers.h.gch

libutil should not depend on libexpr.

https://hydra.nixos.org/build/105961653

It also OOMs.

https://hydra.nixos.org/build/105942679

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

This cuts 'make install -j6' on my laptop from 170s to 134s.