feat(users/sterni/nix/utf8): pure nix utf-8 decoder

users.sterni.nix.utf8 implements UTF-8 decoding in pure nix. We
implement the decoding as a simple state machine which is fed one byte
at a time. Decoding whole strings is possible by subsequently calling
step. This is done in decode which uses builtins.foldl' to get around
recursion restrictions and a neat trick using builtins.deepSeq puck
showed me limiting the size of the thunks in a foldl' (which can also
cause a stack overflow).

This makes decoding arbitrarily large UTF-8 files into codepoints using
nix theoretically possible, but it is not really practical: Decoding a
36KB LaTeX file I had lying around takes ~160s on my laptop.

Change-Id: Iab8c973dac89074ec280b4880a7408e0b3d19bc7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2590
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/nix/int/default.nix

@@ -97,6 +97,8 @@ let
   # i. e. they truncate towards 0
   mod = a: b: let res = a / b; in a - (res * b);
 
+  inRange = a: b: x: x >= a && x <= b;
+
 in {
   inherit
     maxBound
@@ -117,5 +119,6 @@ in {
     bitXor
     toHex
     fromHex
+    inRange
     ;
 }