chore(sterni/exercises/aoc): split out utils into a library of sorts

Change-Id: Id2bb764fcdcd4b345eed6a98e0d67fcb18fc1353
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7488
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/exercises/aoc/lib.bqn

@@ -0,0 +1,8 @@
+IsAsciiNum ⇐ ('0'⊸≤∧≤⟜'9')
+
+ReadInt ⇐ {(𝕨⊸×+⊣)´∘⌽-⟜'0'𝕩} # stolen from leah2
+ReadDec ⇐ 10⊸ReadInt
+
+SplitOn ⇐ ((⊢ (-1˙)⍟⊣¨ +`∘(1⊸»<⊢))∘(≡¨)⊔⊢)
+
+_fix ⇐ {𝕩 𝕊∘⊢⍟≢ 𝔽 𝕩}