feat(users/sterni/nix): add sternis nix lib

What you see here is mostly the fallout of me implementing a correct
urlencode implementation in nix for Profpatsch's blog implementation
(although they'll probably keep it at arm's length).

Where I want to go from here:

* Extend this library towards general purpose nix™, mainly by
  implementing missing interfaces which you'd still have to use
  <nixpkgs/lib> for right now. Reexposing parts of <nixpkgs/lib>
  with better naming is fine for now, at some point I'd contemplate
  making this depend on nothing outside of depot, maybe even itself
  (should be easy we only use yants for an easily replaceable check).

* Improve error messages possibly by carefully reintroducing yants. I
  originally typed essentially everything using yants, but turns out
  this can a) be dangerous when stuff you are handling throws because
  type checking means evaluating and b) has a incredible performance
  cost in some cases.

* Reexpose builtins with better naming and slightly wrapped so they
  don't unrecoverably throw in cases where a null or something would
  suffice.

Change-Id: I33ab08ca4e62dbc16b86c66c653935686e6b0e79
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2541
Reviewed-by: sterni <sternenseemann@systemli.org>
Reviewed-by: Profpatsch <mail@profpatsch.de>
Tested-by: BuildkiteCI

users/sterni/nix/string/default.nix

@@ -0,0 +1,71 @@
+{ depot, lib, ... }:
+
+let
+
+  inherit (depot.users.sterni.nix.char)
+    chr
+    ord
+    ;
+
+  inherit (depot.users.sterni.nix.flow)
+    cond
+    ;
+
+  take = n: s:
+    builtins.substring 0 n s;
+
+  drop = n: s:
+    builtins.substring n (builtins.stringLength s - n) s;
+
+  charAt = i: s:
+    let
+      r = builtins.substring i 1 s;
+    in if r == "" then null else r;
+
+  charIndex = char: s:
+    let
+      len = builtins.stringLength s;
+      go = i:
+        cond [
+          [ (i >= len) null ]
+          [ (charAt i s == char) i ]
+          [ true (go (i + 1)) ]
+        ];
+    in go 0;
+
+  toChars = lib.stringToCharacters;
+  fromChars = lib.concatStrings;
+
+  toBytes = str:
+    builtins.map ord (toChars str);
+
+  fromBytes = is: lib.concatMapStrings chr is;
+
+  pad = { left ? 0, right ? 0, char ? " " }: s:
+    let
+      leftS = fromChars (builtins.genList (_: char) left);
+      rightS = fromChars (builtins.genList (_: char) right);
+    in "${leftS}${s}${rightS}";
+
+  fit = { char ? " ", width, side ? "left" }: s:
+    let
+      diff = width - builtins.stringLength s;
+    in
+      if diff <= 0
+      then s
+      else pad { inherit char; "${side}" = diff; } s;
+
+in {
+  inherit
+    take
+    drop
+    charAt
+    charIndex
+    toBytes
+    fromBytes
+    toChars
+    fromChars
+    pad
+    fit
+    ;
+}