flake: Update inputs

rtl8723bu: 2020-09-04 -> 2020-11-24

flake: Expose lib as an output

For some reason I thought this triggered an error during `nix flake
check`.  Turns out it only emits a warning, so we can now properly
access `lib` without having to import it ourselves now.

wireguard-systemd: Use new functions in our lib

`concatMapAttrs` with Nix' null attribute name erasure is really handy.

wireguard-systemd: Escape interface name in device unit name

I don't use the script options so I hadn't noticed it before, but
escaping is needed for them to work with interfaces with
non-alphanumeric characters in their names.

lib: Add attrsets.{concatMapAttrs,concatMapAttrsToList}

ipfs-cluster: Use more systemd features to ease new cluster setup

ipfs-cluster: Use secret file in new clusters

lib: Add math.rem

math.rem computes the remainder of a truncated division, in which the
sign of the remainder is that of the dividend.  It is the most popular
implementation of the % operator, according to Wikipedia [1].

I thought about writing a math.mod as well, but it seems that there are
several competing alternatives to that.  Knuth's floored division seems
popular, as well as Euclidean division.  So I'll add it when there is
actual need for it.

[1]: https://en.wikipedia.org/wiki/Modulo_operation#In_programming_languages

guile-json: 4.3.2 -> 4.4.1; fix evaluation in restricted mode

matrix-appservice-irc: Fix build on macOS

Drop NixOS 20.03 support

NixOS 20.03 was EOL'ed on November 27, 2020.

As a side effect we can now use the SRI hashes nixUnstable produces
directly instead of converting them to Base32 first.

matrix-appservice-irc: 0.22.0 -> 0.23.0

prometheus-bird-exporter: init at 1.2.4

lib: Add strings.parseHex

lib: Add math.abs and math.pow

Try a new approach to setting Guile load paths

Don't touch the install paths, but let Guile's setup hook do the right
think[^1].  Also run `haunt --version` after installing to make sure we
got this right.

[^1]: Which it somewhat doesn't; see github:NixOS/nixpkgs#88720.

bird-lg: remove

Moved to dn42.nix along with other dn42 services.

bird-lg: Fix import in config loader script

bird-lg: Add option to log to syslog instead of stderr

bird-lg: Expand and fix default settings